Glossary

The standard self-diagnosis system on gasoline cars and light trucks sold in the US since the 1996 model year (diesels followed shortly after). It watches the engine, transmission and emissions systems and stores a trouble code when something is out of range. ABS and airbag codes are read through the same port but aren't part of the OBD-II standard itself. You read it through a connector under the dashboard, usually near the steering column.

A five-character code such as P0420 that the car stores when it finds a fault. The first letter is the system (P powertrain, B body, C chassis, U network), and the rest narrows down the fault. A code tells you where to look, not which part to replace.

Example codes: P0420, P0300

The amber engine-shaped light on the dashboard, officially the malfunction indicator lamp. Steady means a fault is stored; flashing means a misfire bad enough to damage the catalytic converter, so slow down and stop soon.

Example codes: P0300

A pending code means a test failed once and the computer is waiting to see it again. If it fails again on a later drive, the code is confirmed and the check engine light comes on. Pending codes are an early warning.

A snapshot the engine computer saves when it sets a code: engine speed, vehicle speed, temperature, fuel trims and more at that moment. A scanner can show it, and it helps a mechanic recreate the conditions of the fault.

Self-tests the car runs on its emissions systems, such as the catalyst, oxygen sensors and EVAP system. After codes are cleared or the battery is disconnected they reset to "not ready", and many inspection stations reject a car until enough of them have run again.

Example codes: P0420, P0456

The mix of cold start, idling, city and highway driving a car needs before it can rerun its self-tests. It's why a fixed problem can take several days of normal driving before the light goes off and the monitors are ready.

On V-shaped and flat engines, the two rows of cylinders. Bank 1 is the side with cylinder 1; bank 2 is the other side. Inline engines have only bank 1.

Example codes: P0171, P0174

The position of an oxygen sensor in the exhaust. Sensor 1 sits before the catalytic converter and controls the fuel mixture; sensor 2 sits after it and checks how well the converter works.

Example codes: P0420

How much the engine computer is adding or removing fuel to keep the mixture right. Short-term trim reacts instantly; long-term trim is the learned correction. Large positive trims mean the engine is running lean, large negative trims mean it's running rich.

Example codes: P0171, P0172

Lean means too much air for the fuel, often from a vacuum leak or weak fuel delivery; it can cause hesitation and makes the engine run hotter. Rich means too much fuel, often from a leaking injector or a stuck purge valve; it wastes fuel and can overheat the catalytic converter. Both can lead to misfires.

Example codes: P0171, P0172, P0174

A cylinder that doesn't burn its fuel properly on some strokes, felt as shaking, a rough idle or hesitation. Worn spark plugs and failing ignition coils are the most common causes.

Example codes: P0300, P0301

The part in the exhaust that turns harmful gases into less harmful ones. Misfires and oil burning can overheat or poison it, so fix those first before replacing it.

Example codes: P0420, P0430

A sensor in the exhaust that measures leftover oxygen, which tells the computer whether the mixture is lean or rich. The sensor after the converter is used to judge the converter itself.

Example codes: P0420

A sensor in the intake duct that measures how much air enters the engine. A dirty sensor or an air leak after it makes the computer miscalculate fuel.

Example codes: P0101, P0171

The manifold absolute pressure sensor, which measures pressure or vacuum in the intake manifold. Engines use it to work out engine load, and turbo engines use it to check boost.

Example codes: P0106

The system that traps gasoline vapor from the fuel tank in a charcoal canister and later lets the engine burn it. Leaks in it, most often a loose gas cap, set EVAP codes but usually don't affect how the car drives.

Example codes: P0456, P0442, P0455

The two valves that control the EVAP system. The purge valve lets fuel vapor into the engine; the vent valve opens the canister to fresh air. A purge valve stuck open can make the engine run rich and hard to start after refueling.

Example codes: P0441, P0172

Exhaust gas recirculation: a valve that feeds a little exhaust back into the intake to lower combustion temperatures. Carbon buildup that blocks it can cause pinging and emissions codes.

Example codes: P0401

A system that shifts camshaft timing while the engine runs, using engine oil pressure through an oil control valve and a cam phaser. Low or dirty oil is a common reason for timing codes.

Example codes: P0011, P0010

The chain that keeps the camshafts in step with the crankshaft. When it stretches, cam and crank sensors disagree, and on many engines a chain that jumps a tooth can cause serious engine damage.

Example codes: P0016, P0008

Sensors that tell the computer engine speed and which cylinder is about to fire. A missing crankshaft signal can make the engine stall or refuse to start.

Example codes: P0335, P0340

On modern cars the gas pedal is a sensor, and a motor opens the throttle plate. Faults in the pedal sensor, the throttle body or their wiring usually put the car in reduced-power mode.

Example codes: P2111, P2135

A protective backup mode the car switches to after a serious fault: engine power is limited or the transmission holds one gear. It's meant to get you off the road or to a shop, not for normal driving.

Example codes: P2106, P0700

The control module that runs the engine, and on many cars the transmission too (then called the PCM, powertrain control module). Many of its faults come from low voltage, bad grounds or software rather than a dead computer.

Example codes: P0606, U0100

The computer that controls an automatic transmission's shifts and pressures. Basic code readers show its generic P07xx codes, but manufacturer-specific transmission codes and live data need a scanner that can talk to the transmission module. On some cars only a general code such as P0700 shows up on a basic reader.

Example codes: P0700, U0101

The network the car's modules use to talk to each other. U codes mean a module stopped hearing from another one or got data that makes no sense; weak batteries, bad grounds and wiring are common causes.

Example codes: U0100, U0073

The anti-lock braking system, which stops the wheels from locking in a hard stop. When it has a fault, normal brakes usually still work but ABS, traction and stability control switch off.

Example codes: C0020, U0121

The supplemental restraint system: airbags, seat belt pretensioners, crash sensors and the occupant sensor. Its codes need a scanner that reads the airbag module, and its parts should only be handled by a trained shop.

Example codes: B0020, B00A0