Fault codes by system
298 powertrain codes in one list are impossible to browse without knowing the code. Here the same list is split by meaning.
Fuel and air metering, general
Codes about the mixture in general and about auxiliary emission systems: valve timing, oxygen sensor heaters, crankcase ventilation.
Intake measuring sensors
The intake measuring sensors: how much air came in, at what temperature and under what pressure. The coolant temperature sensor and the fuel rail pressure sensor live here too — the unit leans on them when calculating the mixture.
Oxygen sensors
Oxygen sensors before and after the catalyst, their heaters and a stuck signal. They tell the unit what came out of the mixture — and the verdict on catalyst efficiency depends on them too.
Throttle and accelerator pedal
The throttle body, its actuator and the accelerator pedal. There is no cable from pedal to throttle on a modern car: the pedal is a pair of sensors, and a motor opens the flap on the unit's command. If the sensors disagree, the unit refuses to open the throttle.
Mixture: lean and rich
Lean and rich mixture. The best known codes grow from here — P0171 and P0172 — while the P21xx codes state the operating mode and the bank: lean only at idle means an air leak, lean only off idle means delivery.
Fuel and air metering: injectors
Injectors and their circuits: stuck, clogged or not answering the command. They often sit alongside a misfire in the very same cylinder.
Ignition system and misfires
Misfires and everything that causes them. One thing matters most here: if the light is not steady but FLASHING, do not drive on — the catalyst melts in minutes.
Exhaust gas recirculation (EGR)
Exhaust gas recirculation: the EGR valve, its actuator and position sensor. Part of the exhaust is returned to the intake to lower the combustion temperature — and soot returns with it, which is what coats the valve.
Secondary air injection
Secondary air injection: the pump that blows air into the exhaust for the first minute or so after a cold start, so the catalyst reaches working temperature sooner. The rest of the time it does nothing — and that is normal.
Catalytic converter
The catalyst and its efficiency. The most deceptive conclusion in the whole reference: the unit does not measure the state of the catalyst directly — it compares two oxygen sensors and draws a conclusion. So the culprit can be a sensor, an exhaust leak, or even the driving pattern.
Canister and purge valves
The charcoal canister and its valves: the system that catches petrol vapour from the tank and sends it into the engine instead of the atmosphere. The purge valve and the vent valve live here — two parts that stick more often than they fail.
Fuel vapour leaks
Fuel vapour leaks and the pressure sensor that finds them. The car tests the tank for tightness itself — it seals the system and watches the pressure. If it does not add up, a leak code appears, and its size is in the name of the code: large, small or very small.
Fuel level in the tank
The fuel level sender in the tank — the very float the gauge needle is drawn from. It rarely lies about anything important for the engine, but without a correct level the car cannot test the vapour system, which means it is not ready for the emissions test.
Diesel particulate filter
The diesel particulate filter: the differential pressure sensor, regeneration and soot overload. Half the codes in this group are not about hardware but about the driving pattern: the filter cleans itself, but only with a warm exhaust and at speed.
Idle and auxiliary systems
Idle, vehicle speed, cruise control and auxiliary systems. This is also where P1000 lives — the readiness code, which is not a fault at all.
Control module and its circuits
The control unit itself, its power supply and circuits. These codes often arrive in a bunch after a flat battery or someone else's work on the wiring.
Transmission
The gearbox: solenoids, speed sensors, torque converter. For half of these codes the first move is cheap — the fluid level and condition and the transmission connector on the outside.
Communication between modules
U codes: the modules in the car have stopped hearing each other. This is almost never fried electronics — usually a terminal, a ground or a flat battery.
Brakes, ABS and stability control
C codes: brakes, ABS, the stability system and its sensors. Ordinary OBD-II does not read them on every car — but knowing what a code you were told in the workshop means is always useful.
Body, airbags and lighting
B codes: body electronics, airbags, seat belts, lighting and climate control. ⛔ The airbag group is not to be read or cleared with random software: the price of a mistake is an airbag that does not deploy.