OBD Codes- Bank 1, bank 2
- On engines with two rows of cylinders (V6, V8), each row is a bank. Bank 1 is the row that contains cylinder 1. Engines with one row have only bank 1.
- Sensor 1, sensor 2
- The position of an oxygen sensor in the exhaust. Sensor 1 is before the catalytic converter (upstream). Sensor 2 is after it (downstream).
- Lean
- Too much air for the fuel being burnt. Causes hesitation and can overheat the engine.
- Rich
- Too much fuel for the air. Wastes fuel, makes black smoke and can damage the catalytic converter.
- Fuel trim
- The correction the computer makes to the amount of fuel, shown as a percentage. Short-term trim changes second by second. Long-term trim is the learnt average. Large positive trim means the computer is adding fuel to fight a lean condition.
- MIL
- Malfunction indicator lamp: the proper name for the check engine light.
- DTC
- Diagnostic trouble code: the proper name for a fault code.
- Circuit low, circuit high
- The voltage on a sensor wire is below or above the range the computer accepts. Low often means a short to earth or a broken supply. High often means a broken wire or earth.
- Range or performance
- The signal is electrically present but does not make sense, for example a temperature that never changes as the engine warms up.
- Intermittent
- A signal that drops out now and then, usually from a loose plug or a wire broken inside its insulation.
- Freeze frame
- A snapshot of engine speed, temperature, load and other values stored at the moment a fault was recorded.
- Pending code
- A fault seen on one trip only. It becomes a stored code if it happens again.
- Readiness monitor
- One of the self-tests the computer runs on an emission system. After codes are cleared, each monitor must run again before it shows "ready".
- Drive cycle
- The mix of starting, idling and steady driving that lets all readiness monitors run.
- Limp mode
- A protective mode in which the computer limits power or holds one gear so you can reach a workshop without causing damage. Also called limp-home mode.
- ECM, PCM, ECU
- Names for the engine computer: engine control module, powertrain control module, engine control unit.
- TCM
- Transmission control module: the computer of an automatic gearbox.
- CAN bus
- The pair of wires over which the computers of the car exchange data.
- Catalytic converter
- A unit in the exhaust that turns harmful gases into less harmful ones. It contains precious metals and is costly to replace.
- EGR
- Exhaust gas recirculation: feeding some exhaust back into the engine to cut nitrogen oxides.
- EVAP
- The evaporative emission system, which stops petrol vapour escaping from the tank.
- DPF
- Diesel particulate filter: traps soot from a diesel exhaust and burns it off from time to time.
- SCR
- Selective catalytic reduction: a diesel exhaust treatment that uses a urea fluid, sold as AdBlue or diesel exhaust fluid.
- Generic code
- A code defined by the standard, the same on every make. For P codes these are P0xxx, P2xxx and P34xx to P39xx.
- Maker-specific code
- A code whose meaning the car maker chooses, such as P1xxx. It differs between brands.
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