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ASE L1 practice questions

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Question 1General Powertrain Diagnosis

An engine has one cylinder with low compression. After about 15 mL of engine oil is added to that cylinder and the test is repeated, the reading rises to near normal. Which condition is indicated?

  1. Worn piston rings and cylinder wall Correct answer.
  2. A leaking intake valve
  3. A blown head gasket
  4. A collapsed hydraulic lifter

Explanation

Oil added to the cylinder temporarily seals the gap between the rings and the cylinder wall, so a pressure increase after oiling points to the rings as the leak path. Valve and head gasket leaks are not sealed by oil and would not improve.

Question 2General Powertrain Diagnosis

During a cylinder leakage test, air is heard escaping from the tailpipe. Which is the source of the leak?

  1. Head gasket at a coolant passage
  2. Piston rings
  3. Intake valve
  4. Exhaust valve Correct answer.

Explanation

Air that reaches the tailpipe has to pass the exhaust valve and its port, making the exhaust valve the leak path. Each leak path has a distinct listening point, which is how the test localizes the fault.

Question 3General Powertrain Diagnosis

A vehicle produces a steady blue-gray haze from the tailpipe that gets heavier when the engine is accelerated after a long idle. Which condition causes this?

  1. Engine oil being burned Correct answer.
  2. Coolant entering the cylinders
  3. Excess fuel from leaking injectors
  4. Air trapped in the cooling system

Explanation

Burning oil produces blue-gray smoke, and it is heaviest right after idle because oil that seeped past valve guides or turbo seals is burned off when the engine is accelerated. Smoke color and odor separate oil, coolant, and fuel sources.

Question 4General Powertrain Diagnosis

A front-wheel-drive vehicle makes a rhythmic clicking noise from one front wheel that is louder during turns in one direction. Which component is most likely faulty?

  1. A front wheel bearing
  2. A worn outer CV joint Correct answer.
  3. A tire with cupped tread
  4. A loose tie rod end

Explanation

A worn outer constant-velocity joint clicks as it transmits torque while operating at an angle, which is why the noise is loudest in turns. A failing bearing produces a growl or hum that changes with load direction instead.

Question 5General Powertrain Diagnosis

An engine shows 12 psi of oil pressure at a hot idle and 45 psi at 2500 rpm, while the specification calls for a minimum of 20 psi at hot idle. Which is the most likely cause?

  1. Worn main and rod bearings Correct answer.
  2. A stretched timing chain
  3. A leaking intake manifold gasket
  4. A restricted PCV valve

Explanation

Pressure that is low at idle but acceptable at speed shows too much oil is escaping through a large internal clearance, because pump output rises with speed while the leak stays constant. Worn main and rod bearings are the usual cause of that pattern.

Question 6General Powertrain Diagnosis

A vacuum gauge connected to the intake manifold reads 17 in-Hg at idle and slowly falls to about 5 in-Hg when engine speed is held at 2500 rpm. Which condition is indicated?

  1. A restricted exhaust system Correct answer.
  2. Worn piston rings
  3. A leaking brake booster check valve
  4. A leaking intake manifold gasket

Explanation

As exhaust flow increases, backpressure builds and residual exhaust dilutes the incoming charge, so manifold vacuum keeps falling as rpm rises. Vacuum leaks and ring wear produce a steady low or fluctuating reading that does not continue to drop with speed.

Question 7General Powertrain Diagnosis

A dull, deep knock is heard at half crankshaft speed. It is loudest when the engine is lugged at low rpm under load and disappears when the injector for that cylinder is disabled. Which is the most likely cause?

  1. Piston slap
  2. A worn rod bearing Correct answer.
  3. An exhaust manifold leak
  4. A collapsed hydraulic lifter

Explanation

Rod bearing noise occurs once per crankshaft revolution, which is half the firing frequency, and it is load sensitive. Removing the load by cutting fuel for that cylinder silences it.

Question 8General Powertrain Diagnosis

A vehicle shudders at 35 to 45 mph under light acceleration. The shudder disappears when the torque converter clutch is commanded off with a scan tool. Which is the most likely cause?

  1. An out-of-balance driveshaft
  2. Converter clutch shudder Correct answer.
  3. A worn inner CV joint
  4. Cupped tire tread

Explanation

If the symptom vanishes the moment the clutch is released, the clutch lining itself is the source of the shudder. A tire or driveline fault would remain regardless of clutch state.

Question 9General Powertrain Diagnosis

After warm-up, the upper radiator hose is hot and hard while the lower hose stays cool, and the engine runs hot at highway speed but is normal in traffic. Which is the most likely cause?

  1. A thermostat stuck open
  2. An inoperative cooling fan
  3. A restricted radiator Correct answer.
  4. A leaking head gasket

Explanation

At highway speed airflow through the grille is high and the fan is irrelevant, so overheating then points to poor coolant flow through the core. A cool lower hose confirms that hot coolant is not moving through the radiator.

Question 10General Powertrain Diagnosis

Long-term fuel trim is +25% at idle and falls to about +4% at 2500 rpm. Which is the most likely cause?

  1. A restricted exhaust
  2. An intake vacuum leak Correct answer.
  3. Leaking fuel injectors
  4. A weak fuel pump

Explanation

A vacuum leak adds a roughly fixed amount of unmetered air, which is a large share of the charge at idle but a small share at high airflow. The correction therefore shrinks as rpm rises.

Question 11General Powertrain Diagnosis

With the engine idling, the PCV valve is removed from the valve cover grommet; there is no vacuum at the valve and idle quality does not change. Which is the most likely cause?

  1. A plugged PCV passage Correct answer.
  2. A leaking valve cover gasket
  3. A stuck-open PCV valve
  4. A failed purge solenoid

Explanation

Removing the valve should expose manifold vacuum and usually changes idle speed and quality. No vacuum signal at all shows the crankcase side of the PCV system is blocked.

Question 12General Powertrain Diagnosis

An engine has a chirp that repeats once per belt revolution at idle. Checking with a straightedge shows the power steering pump pulley is out of plane with the other pulleys. Which is the cause?

  1. A failing alternator bearing
  2. A misaligned pulley Correct answer.
  3. A worn tensioner spring
  4. A glazed belt

Explanation

A once-per-revolution chirp that tracks a pulley sitting out of plane means the belt is slipping sideways at that pulley. Restoring the drive plane alignment is the repair.

Question 13General Powertrain Diagnosis

An engine sets camshaft-to-crankshaft correlation codes, idles poorly, and rattles briefly at cold start. Which is the most likely cause?

  1. Low fuel pressure
  2. A stretched timing chain Correct answer.
  3. A faulty camshaft position sensor
  4. A leaking intake manifold gasket

Explanation

A startup rattle together with a persistent correlation error points to mechanical timing drift between the crank and cam. A sensor fault alone would not create mechanical noise.

Question 14General Powertrain Diagnosis

A vehicle has a growl above 30 mph that becomes louder in right turns and quieter in left turns, and the noise does not change on different road surfaces. Which is the most likely cause?

  1. A worn right CV joint
  2. A cupped right tire
  3. A worn left wheel bearing Correct answer.
  4. A worn left front strut

Explanation

Turning right shifts weight onto the left side, loading the left wheel bearing and increasing its noise. Tire noise is usually sensitive to road surface, which helps separate the two.

Question 15General Powertrain Diagnosis

A road test in 4th gear at 30 mph with the throttle held wide open shows engine rpm rising while vehicle speed does not increase. Which is the most likely cause?

  1. A slipping clutch disc Correct answer.
  2. A worn pilot bearing
  3. A leaking clutch slave cylinder
  4. Worn synchronizer rings

Explanation

Rpm rising without a matching rise in road speed means the clutch cannot transmit the engine torque. A worn or contaminated friction disc is the usual cause.

Question 16General Powertrain Diagnosis

A rear-wheel-drive vehicle has a whine from the rear axle that is heard on acceleration and disappears on deceleration. Which is the most likely cause?

  1. Ring and pinion wear Correct answer.
  2. Worn axle bearings
  3. Worn universal joints
  4. Worn differential side gears

Explanation

The drive side of the ring and pinion teeth is loaded only during acceleration, so a whine heard only then points to gear wear or a poor contact pattern. Coast-side noise would indicate the opposite face of the teeth.

Question 17General Powertrain Diagnosis

A clunk is heard and felt when shifting from Drive to Reverse with the brake applied, and the engine visibly lifts when the throttle is opened against the brake. Which is the most likely cause?

  1. A loose exhaust hanger
  2. A worn outer CV joint
  3. A worn strut bearing plate
  4. A worn engine mount Correct answer.

Explanation

Torque reaction lifts one side of the powertrain, and a torn or collapsed mount allows excessive movement with a clunk at each load reversal. The mount is the component that is supposed to limit that motion.

Question 18General Powertrain Diagnosis

Coolant is slowly lost with no external leak, and combustion gas detector fluid turns from blue to yellow while the engine runs. Which is the most likely cause?

  1. A failed head gasket Correct answer.
  2. A thermostat stuck closed
  3. A leaking water pump seal
  4. An inoperative radiator cap

Explanation

The detector fluid changes color only when hydrocarbons from combustion are present in the cooling system. That proves a leak path between the combustion chamber and a coolant passage.

Question 19General Powertrain Diagnosis

Long-term fuel trim is -18% at idle and cruise, and the exhaust shows black smoke. Fuel pressure and injector pulse width are normal. Which is the most likely cause?

  1. Leaking injectors Correct answer.
  2. A weak fuel pump
  3. A restricted air filter
  4. A vacuum leak

Explanation

Negative trim means the control module is removing fuel because the mixture is rich. Leaking injectors add fuel that was never commanded, so pulse width and pressure still look normal.

Question 20General Powertrain Diagnosis

A cylinder leakage test at top dead center shows 35% leakage on cylinder 1, and bubbles appear in the radiator with the coolant level rising. Which is the most likely cause?

  1. Worn piston rings
  2. A burned exhaust valve
  3. A leaking intake manifold gasket
  4. A head gasket leak into coolant Correct answer.

Explanation

Air entering the cooling system during the test proves a path from the combustion chamber to a water jacket. That path is a failed head gasket or a crack in the casting.

Question 21General Powertrain Diagnosis

A manual transaxle jumps out of 3rd gear under both acceleration and deceleration, while every other gear stays engaged. Linkage and mounts are in good condition. Which is the most likely cause?

  1. A faulty clutch master cylinder
  2. Worn 3rd gear engagement teeth Correct answer.
  3. A worn pilot bearing
  4. A worn clutch disc

Explanation

Gear jump-out happens when the engagement teeth and the hub sleeve that lock the gear in place are worn or rounded, allowing thrust from the helical gears to push the sleeve out of mesh. The fault appears only in the affected gear.

Question 22General Powertrain Diagnosis

A stall test on a vehicle with a known-good engine gives 1850 rpm against a specification of 2200 to 2500 rpm, and the transaxle does not slip in any gear. Which is the most likely cause?

  1. Low transmission fluid level
  2. A slipping stator clutch Correct answer.
  3. A restricted transmission cooler
  4. A worn forward clutch

Explanation

Stall speed is set by the converter's torque multiplication, which depends on the stator redirecting fluid. If the stator one-way clutch slips, multiplication drops and stall speed falls below specification even with a healthy engine and transaxle.

Question 23General Powertrain Diagnosis

A vehicle has a shudder during acceleration from a stop that gets worse while turning and disappears when coasting. Which is the most likely cause?

  1. A warped brake rotor
  2. A clicking outer CV joint
  3. A worn inner CV joint Correct answer.
  4. A worn front wheel bearing

Explanation

The inner joint runs at a plunge angle and carries torque only under power, so a worn tripod or ball assembly there produces a shudder on acceleration that is aggravated by turning. Coasting unloads it and the symptom disappears.

Question 24General Powertrain Diagnosis

A part-time four-wheel-drive vehicle driven on dry pavement binds and chirps in tight turns. All four tires are the same brand and size, but the front pair was replaced recently and the rears are worn. Which is the most likely cause?

  1. A faulty transfer case shift motor
  2. Worn front CV joints
  3. A worn rear differential ring gear
  4. Mismatched tire diameters Correct answer.

Explanation

In part-time four-wheel drive the front and rear driveshafts are locked together, so any difference in rolling circumference must be absorbed by tire slip. A large front-to-rear wear difference produces exactly this bind and chirp in turns.

Question 25General Powertrain Diagnosis

An engine sets lean codes for both banks at idle and cruise. Fuel pressure and volume are within specification, a smoke test shows no external leak, and the airflow sensor reads lower than expected and rises very little as rpm increases. Which is the most likely cause?

  1. A contaminated MAF sensor Correct answer.
  2. A weak fuel pump
  3. A leaking intake manifold gasket
  4. A restricted fuel filter

Explanation

The control module calculates fuel from measured airflow, so a sensor that reports less air than actually enters delivers too little fuel and drives both banks lean. Pressure and volume stay normal because the fuel system itself is sound.

Question 26General Powertrain Diagnosis

A compression test is performed on a warm engine with the throttle held open. All four cylinders read about 90 psi. When 10 ml of engine oil is added to each cylinder and the test is repeated, all four readings rise to about 130 psi. What do these results indicate?

  1. A blown head gasket
  2. Worn piston rings Correct answer.
  3. Collapsed valve lifters
  4. Leaking intake valves

Explanation

Adding oil temporarily seals the gap between the rings and the cylinder wall, so pressure rises when ring sealing is the problem. A large rise on every cylinder points to uniform ring wear rather than an isolated valve or gasket fault. Valve or head gasket leakage is not sealed by oil, so the wet reading would barely change.

Question 27General Powertrain Diagnosis

A V6 engine has a sharp ticking noise at the rear of the engine bay that is loudest for the first two minutes after a cold start and almost disappears once the engine warms up. Which is the most likely cause?

  1. A failing ignition coil
  2. A leaking exhaust manifold gasket Correct answer.
  3. A worn timing chain tensioner
  4. A restricted catalytic converter

Explanation

Exhaust components expand as they heat, which closes small gaps and quiets a cold exhaust leak. A manifold gasket leak produces a sharp, rhythmic tick that follows exhaust pulses and fades as the metal grows. Converter restriction, chain tension and coil faults do not create a noise that self-corrects with warm-up.

Question 28General Powertrain Diagnosis

An engine burns oil and emits a puff of blue smoke from the tailpipe on the first start after sitting overnight. The smoke clears within a few seconds and does not return during the day. Which is the most likely cause?

  1. Worn valve stem seals Correct answer.
  2. Worn piston rings
  3. A leaking head gasket
  4. A leaking fuel injector

Explanation

Oil that drains past worn valve stem seals pools on top of the valves while the engine sits. On the first start that oil is drawn into the cylinder and burned, producing a brief blue puff. Ring wear produces continuous smoke, especially under load, rather than a single cold-start puff.

Question 29General Powertrain Diagnosis

An engine lacks power at high rpm. A backpressure gauge is installed in the upstream oxygen sensor hole, and it reads 7 psi at 2500 rpm with the throttle held steady; the specification is 1.5 psi maximum. The reading drops to 0.5 psi as soon as the throttle closes. What is the most likely cause?

  1. A restricted catalytic converter Correct answer.
  2. Retarded ignition timing
  3. A weak fuel pump
  4. A clogged air filter

Explanation

Exhaust backpressure measured ahead of the converter should stay low, generally under about 1.5 psi at a steady 2500 rpm. A reading several times the limit that falls away when flow stops indicates the exhaust is being blocked, most often by a melted or plugged converter core. Fuel supply, intake restriction and timing do not raise measured exhaust pressure.

Question 30General Powertrain Diagnosis

An injector balance test is run on a port-injected engine using a fuel pressure gauge. Each injector is pulsed for the same time and the pressure drop is recorded: cylinders 1, 2 and 3 drop 12 psi, while cylinder 4 drops only 3 psi. What does the low drop on cylinder 4 indicate?

  1. A weak ignition coil on cylinder 4
  2. A low-output fuel pump
  3. A restricted number 4 injector Correct answer.
  4. A sticking number 4 intake valve

Explanation

The balance test compares how much fuel each injector delivers by measuring the resulting pressure drop in a closed fuel rail. A much smaller drop on one injector means that injector flows less fuel than the others, usually from contamination or an electrical fault. A single low reading is not explained by a system-wide pump problem or by ignition and valvetrain faults.

Question 31General Powertrain Diagnosis

A no-crank complaint is investigated. Battery voltage is 12.6 V at rest and 10.9 V while cranking, measured at the battery posts. During the same crank, voltage measured at the starter B+ terminal is 8.4 V. What does this indicate?

  1. A starter drive that is slipping
  2. Excessive internal starter resistance
  3. A battery that is only partly charged
  4. High resistance in the starter cable circuit Correct answer.

Explanation

The battery stays near 10.9 V while the starter terminal sees only 8.4 V, so roughly 2.5 V is being lost between the two points. That loss occurs across cables, terminals or grounds and is the classic sign of high resistance in the feed circuit. Performing the same measurement on the ground side confirms which side is at fault.

Question 32General Powertrain Diagnosis

A turbocharged engine produces only 5 psi of boost where 12 psi is specified. When the wastegate actuator hose is disconnected and the road test is repeated, boost remains at 5 psi. What is the most likely cause?

  1. A restricted air filter element
  2. A leaking intake manifold gasket
  3. A wastegate stuck open Correct answer.
  4. A blow-off valve stuck closed

Explanation

With the actuator hose removed, the wastegate should stay fully closed and boost should climb toward the maximum the turbo can make. If boost does not change, the wastegate is already being held open, either by a seized linkage or a disconnected actuator arm. A stuck-closed blow-off valve would raise, not limit, boost.

Question 33General Powertrain Diagnosis

An engine overheats at idle but runs at normal temperature above 2000 rpm. The heater blows cold at idle and hot at higher rpm. The coolant level is correct, the system holds pressure, the thermostat is new, both radiator hoses heat normally and the cooling fan works. What is the most likely cause?

  1. An air pocket in the cooling system
  2. A radiator with blocked tubes
  3. A water pump impeller loose on its shaft Correct answer.
  4. A thermostat that opens too early

Explanation

Coolant flow depends on pump speed, so a pump that moves little coolant at idle causes overheating and a cold heater that recover as rpm rises. A pressure-tight system with a correct level and a working thermostat leaves pump output as the remaining variable. A separated impeller spins on the shaft without moving coolant until higher rpm helps it catch.

Question 34General Powertrain Diagnosis

A four-cylinder engine has a steady misfire at idle on cylinder 2. Spark is confirmed, injector pulse width is normal and the injector balance test is even across all cylinders. A running compression test on cylinder 2 reads lower than the others and does not rise when the throttle is snapped. What is the most likely cause?

  1. A burned exhaust valve in cylinder 2
  2. Worn piston rings in cylinder 2
  3. A worn camshaft lobe for cylinder 2 Correct answer.
  4. A leaking head gasket at cylinder 2

Explanation

The running compression test measures how well the cylinder breathes while the engine is operating. A reading that stays low and fails to rise on a throttle snap shows the cylinder cannot take in additional air, which points to reduced valve lift from a worn cam lobe or a bent pushrod. Valve, ring and gasket leaks are sealing faults that still allow the running reading to rise with airflow.

Question 35General Powertrain Diagnosis

A technician clears DTCs and freeze-frame data with a scan tool. What happens to the readiness monitors?

  1. Readiness status stays ready until the next key-off.
  2. Only the misfire monitor is reset to not ready.
  3. All non-continuous monitors are reset to not ready. Correct answer.
  4. All monitors stay ready, but pending codes are erased.

Explanation

Clearing fault memory erases stored codes and resets every non-continuous monitor to "not ready" until each one completes its enable criteria again. Continuous monitors re-run almost immediately once the engine is started. The vehicle must then be driven through the appropriate drive cycles before an emissions test.

Question 36General Powertrain Diagnosis

A two-trip DTC fault occurs for the first time on a drive cycle. What does the PCM do?

  1. A pending code is stored and the MIL stays off. Correct answer.
  2. The MIL is commanded on right away.
  3. The code is stored as history and the MIL flashes.
  4. The MIL flashes until the next key cycle.

Explanation

Two-trip faults must be seen on two consecutive driving cycles before the PCM commands the MIL. On the first occurrence the fault is stored as a pending code, which may also capture freeze-frame data. If the fault does not repeat on the next cycle, the pending code is erased.

Question 37General Powertrain Diagnosis

What information does freeze-frame data capture?

  1. The number of drive cycles since the last repair
  2. Pass/fail history for the monitor over the last ten trips
  3. Operating conditions at the moment the fault set Correct answer.
  4. The vehicle VIN and calibration part number

Explanation

Freeze frame stores a snapshot of parameters such as engine speed, load, coolant temperature, vehicle speed and fuel trim at the instant the DTC matured. Technicians use it to reproduce the fault under the same conditions. Typically only one frame is retained per fault.

Question 38General Powertrain Diagnosis

In closed-loop fuel control, which input does the PCM use as its primary feedback?

  1. Intake air temperature sensor
  2. Upstream oxygen sensor Correct answer.
  3. Manifold absolute pressure sensor
  4. Throttle position sensor

Explanation

In closed loop the PCM compares the pre-catalyst oxygen sensor signal to a target and adjusts injector pulse width to hold the mixture near stoichiometry. The correction appears as short- and long-term fuel trim. The other sensors modify the base calculation but do not provide mixture feedback.

Question 39General Powertrain Diagnosis

Which monitor runs continuously whenever the engine is running?

  1. Misfire monitor Correct answer.
  2. Catalyst efficiency monitor
  3. Oxygen sensor heater monitor
  4. Evaporative system monitor

Explanation

The misfire, fuel system and comprehensive component monitors are continuous monitors, running whenever their enable conditions are met. Catalyst, EVAP and oxygen sensor monitors are non-continuous and need specific operating conditions before they run.

Question 40General Powertrain Diagnosis

A stored DTC of P0302 identifies which condition?

  1. Random misfire across all cylinders
  2. Catalyst efficiency below threshold, bank 2
  3. Cylinder 2 misfire detected Correct answer.
  4. Cylinder 2 injector circuit fault

Explanation

In the P03xx series the final digit identifies the misfiring cylinder, so P0302 is a misfire in cylinder 2. P0300 indicates a random or multiple-cylinder misfire. An injector circuit fault would fall in the P02xx circuit code range.

Question 41General Powertrain Diagnosis

A good narrow-band zirconia oxygen sensor on a warm engine should switch between roughly which voltages?

  1. 0.0 and 0.3 volt
  2. 0.5 and 1.5 volts
  3. 2.0 and 4.5 volts
  4. 0.1 and 0.9 volt Correct answer.

Explanation

A narrow-band zirconia sensor swings from about 0.1 volt when lean to about 0.9 volt when rich, with a sharp transition near stoichiometry. It does not measure air/fuel ratio linearly. Wide-band sensors are used when a linear ratio signal is required.

Question 42General Powertrain Diagnosis

After the MIL has been commanded on, what normally causes the PCM to turn it off?

  1. Forty warm-up cycles with no fault
  2. One fault-free driving cycle
  3. Disconnecting the battery for ten minutes
  4. Three consecutive fault-free driving cycles Correct answer.

Explanation

Once a two-trip fault has illuminated the MIL, the PCM extinguishes it after three consecutive driving cycles in which the same fault does not recur. The stored DTC and freeze frame remain available until cleared or until the code ages out of memory.

Question 43General Powertrain Diagnosis

A misfire severe enough to damage the catalytic converter causes the PCM to do what?

  1. Flash the MIL once per second Correct answer.
  2. Reduce fuel pressure at the rail
  3. Turn the MIL on steady and store a history code
  4. Disable the fuel injectors on the affected bank

Explanation

A catalyst-damaging (type A) misfire commands the MIL to flash once per second while the misfire is occurring, warning the driver to reduce load. A less severe misfire simply illuminates the MIL steadily. The flash continues until the misfire stops or the driver shuts the engine down.

Question 44General Powertrain Diagnosis

Which condition typically prevents the EVAP monitor from running?

  1. Barometric pressure near sea level
  2. Fuel tank level at 95 percent Correct answer.
  3. Fuel tank level at 50 percent
  4. Engine coolant at operating temperature

Explanation

The EVAP monitor needs a defined vapor space above the fuel, so it is usually inhibited when the tank is above roughly 85 percent full or below roughly 15 percent full. Other common inhibits include a cold engine, high altitude and extreme ambient temperature. A mid-range fuel level is a normal enable condition.

Question 45General Powertrain Diagnosis

The catalyst efficiency monitor compares which two signals?

  1. Manifold pressure and throttle position
  2. Fuel tank pressure and purge duty cycle
  3. Intake and exhaust cam position
  4. Upstream and downstream oxygen sensors Correct answer.

Explanation

The monitor compares switching activity of the post-catalyst oxygen sensor with that of the pre-catalyst sensor. A healthy catalyst stores and releases oxygen, so the downstream signal should be far less active than the upstream signal. When the downstream sensor begins switching as fast as the upstream one, the catalyst is judged degraded.

Question 46General Powertrain Diagnosis

A vehicle sets P0016, crankshaft/camshaft position correlation. Which is the most likely cause?

  1. A jumped or stretched timing chain Correct answer.
  2. A leaking intake manifold gasket
  3. A failed knock sensor
  4. A contaminated mass airflow sensor

Explanation

P0016 means the PCM sees the camshaft and crankshaft signals outside the expected relationship for that engine. Mechanical timing error such as a stretched chain, a jumped tooth or a faulty phaser is the usual cause. Sensor circuit faults are checked first before mechanical diagnosis.

Question 47General Powertrain Diagnosis

Which sequence best describes the elements of a generic OBD II drive cycle?

  1. Cold start, idle, cruise, acceleration and decel Correct answer.
  2. Key on, wide-open throttle for two minutes, key off
  3. Idle for thirty minutes with the A/C on high
  4. Stationary high idle with the brake applied

Explanation

A generic drive cycle is built to run as many monitors as possible by covering cold start and idle, steady cruise, acceleration and deceleration phases. The exact profile varies by manufacturer, so the OEM procedure should be followed. Monitors run only inside their enable criteria, not simply because the engine is running.

Question 48General Powertrain Diagnosis

A properly terminated high-speed CAN bus measures what resistance across DLC pins 6 and 14 with the power off?

  1. About 2.5 ohms
  2. About 120 ohms
  3. About 60 ohms Correct answer.
  4. About 240 ohms

Explanation

High-speed CAN uses two 120-ohm terminating resistors in parallel, which reads about 60 ohms at the diagnostic connector. A reading near 120 ohms suggests one terminator is missing or one leg of the bus is open. A very low reading points to a short between the bus lines.

Question 49General Powertrain Diagnosis

On a scan tool data list, a PID is best described as what?

  1. A procedure for clearing adaptive memory
  2. A permanent code in non-volatile memory
  3. A parameter ID for a live data value Correct answer.
  4. A two-trip fault confirmation counter

Explanation

PID stands for parameter identification; each PID is a numbered data channel broadcast over the diagnostic link, such as engine speed, coolant temperature or calculated load. Scan tools display and graph these live values. They are data identifiers, not fault codes or service procedures.

Question 50General Powertrain Diagnosis

After replacing or cleaning an electronic throttle body, why must a relearn procedure be performed?

  1. To reset the radio and clock memory settings
  2. To relearn the throttle plate rest positions Correct answer.
  3. To initialize the fuel level sender
  4. To clear permanent misfire codes

Explanation

The PCM learns the throttle plate's closed and spring-loaded rest positions so it can control idle speed and limp-home operation accurately. Deposits or a replacement throttle body change those positions, and without a relearn idle quality and pedal response can be poor. Some vehicles relearn automatically during a specified drive cycle.