ASE L1 key terms

An alphanumeric code stored by the powertrain control module when a monitored circuit or system falls outside its expected range. The first character identifies the system, and the remaining characters narrow down the fault.

A snapshot of sensor values and operating conditions that the control module records the moment a fault is first detected. Technicians use it to recreate the conditions that triggered the code.

Internal self-tests the control module runs on emission-related systems. A monitor is ready once its test has run during a drive cycle, and an emissions test is usually rejected if too many are incomplete.

Short-term and long-term corrections the control module applies to injector pulse width based on oxygen sensor feedback. Positive numbers mean fuel is being added to correct a lean condition; negative numbers mean fuel is being removed.

The chemically ideal air-to-fuel mass ratio at which all the fuel and oxygen are consumed, roughly 14.7 to 1 for gasoline. It is the target the control module aims for in closed loop.

A sensor that measures the mass of air entering the engine, usually by heating a wire or film and reading the current needed to keep it hot. The module uses the signal to calculate fuel delivery.

A sensor that reports pressure inside the intake manifold relative to a perfect vacuum or to atmosphere. It lets the module infer engine load and, on speed-density systems, calculate airflow.

A potentiometer or Hall-effect sensor that reports throttle plate angle to the control module. The signal shows how much load the driver is requesting and influences fuel, spark, and shift scheduling.

A sensor in the exhaust stream that generates a voltage when exhaust oxygen differs from oxygen in outside air. The module uses its switching pattern to correct fuel delivery and to judge catalyst efficiency.

A sensor that reports the actual air-fuel ratio across a wide range instead of only switching rich to lean. It supports precise fuel control and is tested differently from a conventional oxygen sensor.

The converter's ability to store and release oxygen and to convert hydrocarbons, carbon monoxide, and nitrogen oxides. The module infers it by comparing rear oxygen sensor activity with front sensor activity.

A system that routes a small amount of exhaust gas back into the intake to lower combustion temperature and reduce nitrogen oxide formation. It also reduces pumping losses and knock at part throttle.

A sealed system that captures fuel vapor from the tank and stores it in a charcoal canister until it can be purged into the intake. Leaks or purge faults set codes and block monitor completion.

A system that uses manifold vacuum to pull crankcase vapors into the intake for burning, preventing pressure buildup and hydrocarbon release. A stuck-open valve creates a vacuum leak; a clogged one causes oil leaks and sludge.

A pump or pulse system that adds fresh air to the exhaust during cold start to speed catalyst warm-up and oxidize carbon monoxide and hydrocarbons. Air is normally diverted or shut off once the engine warms.

A transformer that steps battery voltage up to the thousands of volts needed to jump the spark plug gap. It stores energy in a primary winding and releases it into a secondary winding when the primary circuit opens.

A threaded component with center and ground electrodes that provides the gap where the ignition spark lights the compressed air-fuel mixture. Gap size, heat range, and wear all affect misfire and combustion quality.

The time, measured in milliseconds or distributor degrees, that current flows through the ignition coil primary before the spark fires. Too little dwell gives a weak spark; too much can overheat the coil or module.

A piezoelectric sensor bolted to the engine block that converts structure-borne vibration from detonation into a voltage signal. The module responds by retarding timing to protect the engine.

A distributorless design in which one coil fires two cylinders at the same time, one on compression and one on exhaust. The paired cylinders share a coil and are wired in series.

A sensor that reports crankshaft speed and position to the control module using a reluctor wheel with a magnetic or Hall-effect pickup. Its signal is essential for spark timing, injector timing, and misfire detection.

A sensor that identifies which cylinder is on the compression stroke, enabling sequential fuel injection and precise spark timing. A faulty signal usually causes long crank times or random misfire instead of a no-start.

A measurement of the peak pressure each cylinder produces while cranking with the throttle open and ignition disabled. Low or uneven readings point to worn rings, leaking valves, or a head gasket failure.

A test that applies compressed air to a cylinder held at top dead center with valves closed and measures the percentage escaping. Where the air leaks, whether intake, exhaust, crankcase, or coolant, identifies the failed part.

A reading of intake manifold vacuum, typically about 17 to 22 inches of mercury at idle at sea level. A steady low reading suggests a mechanical problem, while a fluctuating needle points to valve or ignition faults.

Measuring the voltage lost across a conductor or connection while current is flowing. A drop above roughly half a volt on a power or ground path indicates excessive resistance such as corrosion or a loose terminal.

An OBD II strategy that watches crankshaft velocity changes to detect cylinders failing to contribute power. It can set pending, current, and flashing codes, and flashing warns of catalyst-damaging misfire.

A procedure that fires each injector for a fixed pulse and measures the resulting drop in fuel rail pressure. Injectors that produce a smaller pressure drop are restricted or clogged.

A device that maintains a constant pressure difference across the injectors so fuel delivery depends only on pulse width. On returnless systems, regulation is typically handled at the tank by the module-controlled pump.

Measuring exhaust pressure upstream of the catalytic converter while the engine runs, often through a gauge installed in the oxygen sensor hole. High readings indicate a restricted converter or crushed pipe.

A device that communicates with control modules over the diagnostic link to read codes, view live data, run tests, and check monitor status. Its capabilities depend on the tool and the vehicle's communication protocol.

A two-wire differential network that lets control modules exchange data at high speed. Faults can come from a short between the twisted pair, a short to power or ground, or a failed terminating resistor.

The computer that runs fuel, spark, transmission, and emission strategies based on sensor inputs. It stores codes, adapts to wear, and commands outputs such as injectors, coils, and solenoids.

The module's ability to memorize corrections, such as fuel trim and idle airflow, so it can compensate for wear and carbon without relearning every drive. These values are erased when battery power is removed.

A defined sequence of start-up, warm-up, and driving conditions that lets the control module run all of its self-tests. Technicians follow the manufacturer's cycle to set monitors before an emissions inspection.

Measuring hydrocarbons, carbon monoxide, carbon dioxide, oxygen, and oxides of nitrogen in tailpipe gas to diagnose combustion and emissions faults. Carbon dioxide and oxygen readings together show whether the mixture is rich, lean, or diluted.

An emissions inspection in which the vehicle is driven on a dynamometer under simulated load at steady speeds while tailpipe gases are measured. It reflects real driving better than an idle-only test.

A code stored after the first drive cycle in which a fault occurs, before a second cycle confirms it. A pending code often matures into a confirmed code that turns on the malfunction indicator lamp.