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Cert IV Auto Diagnostic study materials
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Cert IV Auto Diagnostic practice questions

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Question 1Diagnostic Testing and Fault Isolation

A scan tool shows a P0300 code. What does this code indicate?

  1. A random or multiple-cylinder misfire has been detected Correct answer.
  2. The upstream oxygen sensor heater circuit has failed
  3. The transmission is slipping in second gear
  4. The fuel tank pressure sensor reads out of range

Explanation

P0300 is the generic powertrain code for random or multiple-cylinder misfire, logged when the crankshaft speed fluctuates beyond the calibrated threshold. Cylinder-specific misfires use P0301 through P0312. The code does not identify ignition, fuel or mechanical cause, so further testing is needed.

Question 2Diagnostic Testing and Fault Isolation

During a compression test on a warm petrol engine, one cylinder reads 60 percent of the others. A wet test raises it close to normal. What does this most likely indicate?

  1. Worn piston rings or cylinder bore Correct answer.
  2. Incorrect valve timing
  3. A burned exhaust valve
  4. A leaking head gasket

Explanation

Adding oil temporarily seals the gap between rings and bore, so a pressure rise on the wet test points to ring or bore wear. Valve and head gasket faults are not sealed by oil because the leak path is outside the ring seal. Valve timing faults affect all cylinders similarly.

Question 3Diagnostic Testing and Fault Isolation

A vehicle has a parasitic draw of 0.9 A after all modules have gone to sleep. Which method best isolates the offending circuit?

  1. Replace the battery and recheck the draw
  2. Measure voltage drop across each fuse while the circuit is live Correct answer.
  3. Run the engine and read alternator output
  4. Disconnect the battery and measure resistance to ground

Explanation

Measuring the millivolt drop across fuses while current flows shows which circuit is drawing current without disturbing the sleeping modules. Resistance testing with the battery disconnected is unreliable because modules may wake and because the meter supplies only a tiny test current.

Question 4Diagnostic Testing and Fault Isolation

An oscilloscope trace of a crankshaft position sensor shows a clean signal at idle but drops out above 2500 rpm. What is the most likely cause?

  1. A failed ignition coil
  2. An intermittent break in the sensor wiring or connector Correct answer.
  3. A clogged air filter
  4. Low fuel pressure at high demand

Explanation

A signal that is clean at low speed and disappears at higher speed commonly indicates a marginal connection or wire that opens under vibration or thermal movement. Fuel and air faults do not interrupt a crank signal. Ignition faults do not normally affect the crank sensor waveform.

Question 5Diagnostic Testing and Fault Isolation

Which tool is used to measure the current flowing through a battery cable without disconnecting it?

  1. A feeler gauge
  2. A clamp meter Correct answer.
  3. A test light
  4. A vacuum gauge

Explanation

A clamp meter uses a Hall-effect or inductive jaw to measure current by sensing the magnetic field around a conductor. This allows current measurement without breaking the circuit. Test lights, vacuum gauges and feeler gauges measure different quantities.

Question 6Diagnostic Testing and Fault Isolation

A vehicle pulls to the right only when the brakes are applied. Which diagnostic step is most appropriate first?

  1. Bleed the entire system and retest
  2. Inspect the brake calipers and hoses on both sides for restriction or seizure Correct answer.
  3. Check the wheel alignment
  4. Replace the master cylinder

Explanation

A pull during braking usually means unequal braking force, often from a sticking caliper piston, collapsed hose or contaminated pads. Inspecting both sides first is the logical step before condemning hydraulic components. Alignment affects pulling while driving, not braking.

Question 7Diagnostic Testing and Fault Isolation

A scan tool reads a fuel trim of +25 percent on both banks at idle, falling to +8 percent at 2500 rpm. What does this suggest?

  1. A weak fuel pump at high flow
  2. A vacuum leak affecting idle mixture Correct answer.
  3. A faulty ignition coil
  4. A restricted exhaust system

Explanation

Positive fuel trim that is high at idle but improves with engine speed is the classic signature of an unmetered air leak, because the leak is a larger proportion of total airflow at low speed. A weak pump produces worse trims under load and high rpm.

Question 8Diagnostic Testing and Fault Isolation

What does a KOEO self-test on an OBD-II system primarily verify?

  1. Electrical continuity and sensor range with the engine off Correct answer.
  2. Transmission shift quality
  3. Ignition timing at wide-open throttle
  4. Fuel pressure under load

Explanation

Key-on engine-off testing checks the electrical integrity of circuits and the plausibility of sensor readings without the engine running. It cannot evaluate dynamic conditions such as fuel delivery under load or shift quality. Those require running tests and road testing.

Question 9Diagnostic Testing and Fault Isolation

A starter motor cranks slowly and the battery voltage drops to 9.2 V during cranking. Battery state of charge and connections are good. What is the next logical test?

  1. Measure voltage drop across the starter circuit under load Correct answer.
  2. Test the fuel injector resistance
  3. Check the alternator belt tension
  4. Inspect the exhaust hangers

Explanation

A slow crank with confirmed good battery and connections points to excessive resistance in the starter circuit. A voltage drop test on the positive and ground paths under cranking load locates where the loss occurs. Alternator, injector and exhaust checks do not address cranking current.

Question 10Diagnostic Testing and Fault Isolation

Which symptom most strongly suggests a failing wheel speed sensor on one corner?

  1. ABS warning lamp with an intermittent speed signal dropout on that wheel Correct answer.
  2. A spongy brake pedal
  3. Uniform brake pedal pulsation
  4. Uneven tyre wear across the tread

Explanation

A wheel speed sensor fault typically logs an ABS code and shows a signal that drops out or reads erratically on one wheel compared with the others. Pedal pulsation relates to rotor runout, and a spongy pedal indicates air in the hydraulics.

Question 11Diagnostic Testing and Fault Isolation

A technician measures 0.4 V drop on the ground side of a headlamp circuit that should be near 0.1 V. What does this indicate?

  1. Normal operation for a halogen lamp
  2. Excessive resistance in the ground path Correct answer.
  3. Too much voltage from the alternator
  4. A short to positive in the lamp circuit

Explanation

Voltage drop on a ground circuit represents resistance between the load and battery negative. A reading significantly above the specified maximum indicates a corroded or loose ground connection. A short to positive would produce different symptoms such as a lamp staying on.

Question 12Diagnostic Testing and Fault Isolation

An engine has a steady misfire on cylinder 3. Swapping the coil and plug to cylinder 2 moves the misfire to cylinder 2. What is the conclusion?

  1. The fault is a mechanical fault in cylinder 3
  2. The fault is in the cylinder 3 wiring harness
  3. The fault is in cylinder 3 injector
  4. The fault is in the coil or plug that was moved Correct answer.

Explanation

When a fault follows a component to a new location, that component is the cause. This swapping technique isolates the faulty part from the cylinder position. If the misfire had stayed on cylinder 3, the injector, wiring or mechanical condition would be suspect.

Question 13Diagnostic Testing and Fault Isolation

Which measurement best confirms a blocked catalytic converter?

  1. Intake manifold vacuum at idle only
  2. Fuel injector pulse width at idle
  3. Exhaust backpressure measured upstream of the converter Correct answer.
  4. Battery voltage during cranking

Explanation

Measuring backpressure upstream of the converter shows the restriction directly; excessive pressure at higher rpm confirms blockage. Vacuum readings can suggest restriction but are less specific. Voltage and injector pulse width do not measure exhaust flow.

Question 14Diagnostic Testing and Fault Isolation

A vehicle sets a P0171 (lean bank 1) code. Which condition is a plausible cause?

  1. A failed thermostat
  2. A leaking intake manifold gasket Correct answer.
  3. A restricted fuel filter
  4. A leaking fuel injector

Explanation

A leaking intake gasket admits unmetered air, causing a lean condition and positive fuel trim. A leaking injector adds fuel and drives trims rich. A restricted filter reduces fuel and can also cause lean, but the gasket is a classic unmetered-air cause; the thermostat affects warm-up.

Question 15Diagnostic Testing and Fault Isolation

During a road test, a scan tool shows the transmission commanded into 3rd gear but the ratio calculated from input and output speed matches 2nd gear. What does this indicate?

  1. Low engine oil level
  2. A worn clutch pedal
  3. An internal transmission slip or stuck shift valve Correct answer.
  4. A faulty vehicle speed sensor only

Explanation

Comparing commanded gear with the actual ratio derived from input and output speed sensors reveals slip or a shift that did not occur. A mismatch means the transmission is not achieving the commanded ratio, pointing to internal hydraulic or mechanical faults.

Question 16Diagnostic Testing and Fault Isolation

What is the purpose of a smoke test in EVAP diagnostics?

  1. To test the alternator diode pattern
  2. To check ignition coil output
  3. To reveal leaks in the evaporative emission system by visible smoke Correct answer.
  4. To measure fuel octane rating

Explanation

A smoke machine introduces low-pressure smoke into the sealed EVAP system so escaping smoke makes leaks visible. It is a leak-location tool, not a measurement of fuel quality or electrical output. This makes it valuable for small leaks that pressure testing may miss.

Question 17Diagnostic Testing and Fault Isolation

A coolant temperature sensor reads 20 degrees C on a scan tool while the engine is at operating temperature. The sensor circuit is suspected. Which test confirms the sensor itself?

  1. Replace the thermostat
  2. Measure fuel pressure
  3. Check the radiator cap pressure
  4. Measure the sensor resistance and compare it with the temperature-resistance chart Correct answer.

Explanation

A negative temperature coefficient coolant sensor has a known resistance for each temperature. Comparing measured resistance against the specification chart determines whether the sensor or the wiring is at fault. Thermostat, cap and fuel pressure do not test the sensor.

Question 18Diagnostic Testing and Fault Isolation

Which condition would cause a high reading on a cylinder leakage test with air escaping from the oil filler cap?

  1. A blown head gasket between cylinders
  2. A leaking exhaust valve
  3. Worn piston rings Correct answer.
  4. A leaking intake valve

Explanation

Air escaping through the crankcase and out the oil filler indicates leakage past the piston rings into the sump. Intake valve leakage escapes through the throttle body, and exhaust valve leakage escapes at the tailpipe. A head gasket leak between cylinders shows bubbles in the cooling system.

Question 19Diagnostic Testing and Fault Isolation

A customer reports the battery goes flat overnight. Which first step is most appropriate?

  1. Replace the alternator
  2. Recharge the battery and return the vehicle
  3. Verify the battery condition and measure key-off current draw Correct answer.
  4. Replace all fuses

Explanation

Confirming battery health and measuring key-off current after modules sleep establishes whether the problem is the battery or a parasitic drain. Replacing parts or fuses before measuring wastes time and may not fix the cause. Recharging alone does not diagnose the drain.

Question 20Diagnostic Testing and Fault Isolation

An ignition coil primary waveform shows an unusually long dwell and a low current ramp. What is the most likely cause?

  1. High resistance in the coil primary circuit Correct answer.
  2. A clogged fuel filter
  3. A shorted secondary winding
  4. A lean fuel mixture

Explanation

High resistance in the primary path limits current rise, so the driver keeps the coil on longer to reach target current. A shorted secondary would show different waveform characteristics, and fuel faults do not alter primary current. Dwell and current are electrical parameters.

Question 21Diagnostic Testing and Fault Isolation

Which component converts a mechanical vibration or rotation into an electrical signal for the ECU?

  1. A relay
  2. A sensor Correct answer.
  3. An actuator
  4. A fuse

Explanation

Sensors translate physical conditions such as speed, pressure or temperature into electrical signals the control module can interpret. Actuators perform actions commanded by the module, while relays and fuses are circuit protection and switching devices.

Question 22Diagnostic Testing and Fault Isolation

A CAN bus has 60 ohms measured between CAN high and CAN low with the battery disconnected. What does this indicate?

  1. Excessive bus capacitance
  2. An open circuit in one terminating resistor
  3. Two 120-ohm terminating resistors are present and the bus is intact Correct answer.
  4. A short between CAN high and CAN low

Explanation

Two 120-ohm terminators in parallel produce 60 ohms, which is the expected reading on a healthy high-speed CAN network. A single failed terminator would read 120 ohms, and a short would read near 0 ohms. This resistance check confirms bus integrity before live testing.

Question 23Diagnostic Testing and Fault Isolation

What does a fuel pressure gauge reading that drops rapidly after the pump stops suggest?

  1. A weak ignition coil
  2. A leaking injector or a faulty check valve Correct answer.
  3. A clogged fuel filter
  4. A faulty oxygen sensor

Explanation

Pressure should hold after pump shutdown if the check valve and injectors seal. A rapid drop indicates fuel escaping past a leaking injector or back through a faulty check valve. A clogged filter would cause low running pressure rather than a rapid bleed-down.

Question 24Diagnostic Testing and Fault Isolation

A technician is diagnosing an intermittent fault and wants to capture data only when the symptom occurs. Which scan tool function is most useful?

  1. A code clear function
  2. A freeze frame delete function
  3. A snapshot or data trigger function Correct answer.
  4. A relearn function

Explanation

Snapshot or trigger functions record a frame of live data when a set condition occurs, capturing intermittent events that may be missed during continuous viewing. Clearing codes or deleting freeze frames destroys evidence. Relearn procedures adapt new components.

Question 25Diagnostic Testing and Fault Isolation

An engine idles rough and has a rhythmic intake popping noise. Which fault is most consistent with this symptom?

  1. A weak fuel pump
  2. A restricted exhaust
  3. A faulty coolant sensor
  4. A leaking intake valve Correct answer.

Explanation

Intake popping is caused by combustion pressure escaping back into the intake during the compression or power stroke, typically from a leaking intake valve or burned seat. Exhaust restriction, fuel delivery and coolant sensing do not produce rhythmic intake noise.

Question 26Diagnostic Testing and Fault Isolation

A technician connects a high-impedance digital multimeter across the two signal wires of a two-wire inductive crankshaft position sensor while cranking. Which reading should be expected?

  1. A fixed 0.5 V with no change during cranking
  2. An alternating voltage that rises with cranking speed Correct answer.
  3. A constant 12 V supply voltage
  4. A steady 5 V reference with no fluctuation

Explanation

A two-wire inductive sensor generates its own AC voltage as the reluctor teeth pass the coil, so amplitude and frequency increase with engine speed. A multimeter set to AC volts should show a fluctuating signal that grows as cranking rpm rises.

Question 27Diagnostic Testing and Fault Isolation

Which instrument is most appropriate for checking whether a fuel injector is receiving a pulsed command from the PCM?

  1. An ohmmeter placed across the injector terminals
  2. A vacuum gauge connected to the intake manifold
  3. A timing light clipped to the number one spark plug lead
  4. A noid light plugged into the injector harness connector Correct answer.

Explanation

A noid light is designed to flash when the PCM pulses the injector ground, confirming command without removing the injector. It directly verifies the electrical switching signal reaching the injector.

Question 28Diagnostic Testing and Fault Isolation

During a charging system test, a scan tool shows the generator's commanded duty cycle at maximum but measured charging voltage stays near battery voltage. Which conclusion is most reasonable?

  1. The voltage reading is normal for a fully loaded system
  2. The generator field circuit or generator itself is likely faulty Correct answer.
  3. The battery is fully charged and the regulator is limiting output
  4. The scan tool is misreporting because duty cycle is irrelevant

Explanation

When the PCM commands full field and output voltage fails to rise, the generator is not responding to command. The fault lies in the field circuit, regulator, or generator rather than in the battery.

Question 29Diagnostic Testing and Fault Isolation

A scan tool shows short-term fuel trim oscillating rapidly between plus and minus values while long-term trim stays near zero. What does this pattern indicate?

  1. The fuel pressure regulator has failed
  2. A vacuum leak is adding unmetered air at idle
  3. The oxygen sensor feedback loop is actively correcting mixture Correct answer.
  4. The engine coolant sensor is skewed cold

Explanation

Rapid short-term trim swings around zero show the closed-loop system is actively adjusting fuel to hold stoichiometry. Long-term trim staying near zero confirms no large persistent correction is needed.

Question 30Diagnostic Testing and Fault Isolation

A technician wants to verify that a suspect relay's coil is being energized by the control module. Which test is most direct?

  1. Measure resistance across the relay's load contacts
  2. Check the fuse rating for the relay circuit
  3. Backprobe the coil control wire and watch for voltage change when the module commands the relay Correct answer.
  4. Measure voltage at the battery terminals with the engine running

Explanation

Monitoring the coil control circuit for the commanded voltage change confirms whether the module is energizing the relay. This isolates control-side faults from relay or load-side faults.

Question 31Diagnostic Testing and Fault Isolation

An ABS warning lamp illuminates intermittently and the scan tool stores a code for an erratic signal from the right rear wheel speed sensor. Which inspection is most appropriate first?

  1. Replace all four wheel speed sensors
  2. Inspect the sensor harness and tone ring at that wheel for damage or debris Correct answer.
  3. Bleed the brake system
  4. Replace the ABS hydraulic control unit

Explanation

An erratic signal at one corner usually points to a damaged harness, loose connector, or contaminated tone ring at that wheel. Physical inspection of that specific circuit is the logical first step.

Question 32Diagnostic Testing and Fault Isolation

A vehicle has a parasitic draw of 850 mA after all modules should have gone to sleep. Which method best isolates the offending circuit?

  1. Replace the battery with a known-good unit
  2. Disconnect the battery and measure its open-circuit voltage
  3. Run the engine and read charging voltage
  4. Measure voltage across each fuse while the draw is present Correct answer.

Explanation

Measuring the small voltage drop across fuses while the draw persists shows which circuit is carrying current, localizing the fault. This avoids pulling every fuse and disturbing the system.

Question 33Diagnostic Testing and Fault Isolation

A compression test on a four-cylinder engine gives readings of 150, 148, 152, and 95 psi. A wet test on the low cylinder raises it to 140 psi. What does this indicate?

  1. A burned exhaust valve
  2. Worn piston rings or cylinder bore Correct answer.
  3. A sticking intake valve
  4. A leaking head gasket between cylinders

Explanation

A large increase on the wet test shows the added oil is sealing the piston rings, so the low reading comes from ring or bore wear. Valve or gasket faults typically show little change when oil is added.

Question 34Diagnostic Testing and Fault Isolation

A scan tool shows a mass airflow sensor reading of 3 g/s at idle on a warm 2.0 L engine, and fuel trims are strongly negative. What is the most likely interpretation?

  1. The engine has a vacuum leak
  2. The fuel injectors are clogged
  3. The MAF sensor is over-reporting airflow Correct answer.
  4. The MAF sensor is under-reporting airflow

Explanation

A low reported airflow with negative fuel trim means the PCM is reducing fuel because the actual mixture is rich relative to the reported air. An over-reporting MAF causes the PCM to add too much fuel, producing negative trims.

Question 35Diagnostic Testing and Fault Isolation

A technician observes a fuel injector current waveform that shows the initial peak but then drops to zero well before the commanded pulse ends. What does this suggest?

  1. The injector coil circuit is open or has high resistance Correct answer.
  2. The fuel pressure is too high
  3. The PCM is commanding a longer pulse than needed
  4. The injector is operating normally

Explanation

The current should hold at a lower level until the pulse ends. An early drop to zero indicates the circuit opened, such as a broken coil winding or poor connection.

Question 36Diagnostic Testing and Fault Isolation

Which test best confirms that a starter motor's ground circuit is not causing slow cranking?

  1. Measure voltage drop from the starter case to battery negative during cranking Correct answer.
  2. Measure resistance of the starter solenoid winding
  3. Measure battery open-circuit voltage
  4. Check the starter drive gear teeth

Explanation

A voltage drop test from the starter case to battery negative during cranking shows whether the ground path is adding resistance. Excessive drop indicates a poor engine or battery ground connection.

Question 37Diagnostic Testing and Fault Isolation

A vehicle with a distributorless ignition system has a misfire on one cylinder. A spark tester at that cylinder shows no spark, but swapping the coil to another cylinder moves the no-spark condition. What is the faulty component?

  1. The spark plug
  2. The PCM
  3. The crankshaft position sensor
  4. The ignition coil Correct answer.

Explanation

When the no-spark condition follows the coil to a different cylinder, the coil itself is the cause. The fault moves with the component, isolating it.

Question 38Diagnostic Testing and Fault Isolation

A technician is checking a three-wire Hall-effect camshaft sensor. With the key on and the connector backprobed, which set of readings is most consistent with a healthy sensor?

  1. Battery voltage on the ground wire
  2. 12 V on the signal wire at all times
  3. 5 V reference present, ground good, and signal toggling between low and high as the engine is cranked Correct answer.
  4. 0 V on all three wires

Explanation

A Hall sensor needs a reference, a good ground, and a signal that switches between low and high as the target passes. Observing the signal toggle during cranking confirms the sensor and circuit are working.

Question 39Diagnostic Testing and Fault Isolation

A CAN bus reads about 120 ohms between CAN high and CAN low with the battery disconnected and the modules asleep. What is the most likely cause?

  1. The bus is shorted to ground
  2. One terminating resistor is missing or the bus is open at one end Correct answer.
  3. Both terminating resistors are present and correct
  4. The bus is shorted to battery positive

Explanation

Two 120 ohm terminators in parallel give about 60 ohms. A reading of about 120 ohms means only one terminator is being measured, suggesting an open or missing terminator.

Question 40Diagnostic Testing and Fault Isolation

A vehicle's data link connector has no power on the battery positive terminal that should supply pin 16. Which step is most appropriate?

  1. Replace the scan tool
  2. Replace the PCM
  3. Perform a smoke test of the EVAP system
  4. Check the fuse and wiring that supply power to the DLC Correct answer.

Explanation

The DLC needs battery power on its designated pin for the scan tool to communicate. A missing supply points to a blown fuse or open wiring feeding the connector.

Question 41Diagnostic Testing and Fault Isolation

During a cylinder balance test, disabling cylinder 2 produces no change in engine speed while disabling the others causes a noticeable drop. What does this indicate?

  1. All cylinders are balanced correctly
  2. Cylinder 2 is the strongest cylinder
  3. The test procedure is invalid
  4. Cylinder 2 is contributing little or no power Correct answer.

Explanation

A cylinder that is already not contributing will show little rpm change when disabled. The lack of change identifies cylinder 2 as the weak or dead cylinder.

Question 42Diagnostic Testing and Fault Isolation

A turbocharged engine has low power and a scan tool shows actual boost well below desired boost, with no intake leaks found. Which component should be inspected next?

  1. The fuel tank pressure sensor
  2. The oxygen sensor heater circuit
  3. The brake light switch
  4. The wastegate actuator or boost control solenoid Correct answer.

Explanation

When boost is below command and the intake is sealed, the boost control system or wastegate may be stuck open or not regulating. Inspecting the actuator and solenoid targets the likely cause.

Question 43Diagnostic Testing and Fault Isolation

A technician suspects a faulty throttle position sensor on a drive-by-wire vehicle. Which pair of signals is most useful to compare on a scan tool?

  1. Fuel level and fuel pressure
  2. APP sensor voltages and TP sensor voltages Correct answer.
  3. Vehicle speed and engine rpm
  4. Battery voltage and charging voltage

Explanation

Drive-by-wire systems use accelerator pedal position sensors and throttle position sensors with correlated signals. Comparing them reveals whether the pedal request and throttle response agree.

Question 44Diagnostic Testing and Fault Isolation

An engine has a rhythmic intake popping noise and rough idle, and a cylinder leakage test on one cylinder shows air escaping from the intake port. Which fault is most likely?

  1. A leaking intake valve Correct answer.
  2. A leaking exhaust valve
  3. A failed head gasket to the coolant jacket
  4. Worn piston rings

Explanation

Air escaping into the intake port during a leakage test means the intake valve is not sealing. The rhythmic popping is combustion pressure escaping back into the intake.

Question 45Diagnostic Testing and Fault Isolation

A vehicle sets a code for an open circuit in the heated oxygen sensor heater. Which measurement should be made first?

  1. Resistance of the sensor heater element and continuity of its supply and ground Correct answer.
  2. Fuel pressure at the rail
  3. Transmission line pressure
  4. Ignition timing at idle

Explanation

An open heater circuit code directs diagnosis to the heater element and its wiring. Checking element resistance and circuit continuity locates the open.

Question 46Diagnostic Testing and Fault Isolation

A scan tool records freeze frame data showing the fault occurred at 60 km/h with engine coolant at 90 degrees C and short-term trim at plus 18 percent. What is the main value of this data?

  1. It confirms the exact component that failed
  2. It shows the operating conditions present when the fault was set Correct answer.
  3. It indicates the battery state of charge
  4. It proves the fault is currently active

Explanation

Freeze frame captures the conditions at the moment the code was stored, helping reproduce the fault. It does not by itself identify the failed component or prove the fault is still present.

Question 47Diagnostic Testing and Fault Isolation

A technician is diagnosing a vibration that occurs only at 80 km/h. Which approach best isolates whether the source is tire or driveline related?

  1. Check engine oil level
  2. Replace all tires and retest
  3. Perform a road test with the vehicle on a lift and note whether the vibration changes Correct answer.
  4. Replace the driveshaft

Explanation

Running the vehicle with wheels off the ground and observing whether the vibration persists helps separate wheel and tire sources from driveline sources. Changing conditions under load or speed further localize the fault.

Question 48Diagnostic Testing and Fault Isolation

A technician observes that a PWM-controlled cooling fan runs at full speed whenever the key is on, even when the engine is cold. Which condition best explains this?

  1. The fan control circuit is shorted to ground, commanding full duty Correct answer.
  2. The radiator cap is weak
  3. The coolant temperature sensor is reading too high
  4. The thermostat is stuck open

Explanation

A short to ground on the fan control wire can apply a continuous full-duty command, running the fan regardless of temperature. This matches the constant full-speed behavior.

Question 49Diagnostic Testing and Fault Isolation

A vehicle with a returnless fuel system shows normal pressure at idle but pressure drops sharply when the engine is snapped to wide-open throttle. Which cause is most likely?

  1. A restricted fuel filter or weak pump Correct answer.
  2. A faulty oxygen sensor
  3. A stuck-open purge valve
  4. A leaking fuel injector

Explanation

A restriction or weak pump can maintain pressure at low demand but fail to keep up when flow demand rises. The drop under snap throttle reveals insufficient volume delivery.

Question 50Diagnostic Testing and Fault Isolation

A technician needs to confirm whether an intermittent no-crank condition is on the control side or the power side of the starter circuit. Which test is most useful?

  1. Measure battery voltage at rest
  2. Inspect the starter mounting bolts
  3. Measure voltage at the starter solenoid control terminal while a helper turns the key Correct answer.
  4. Check the battery's reserve capacity rating

Explanation

Monitoring voltage at the solenoid control terminal during a crank attempt shows whether the command reaches the starter. Presence or absence of voltage separates control-side faults from power-side faults.