Fault code hub

Every code has an official definition. Almost none of them tell you what to actually do. Here's the plain-English version, grouped by what's actually going on under the bonnet.

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41 common codes, in plain English.

Engine and misfires

P0300 The engine is misfiring on multiple or random cylinders. P0325 The knock sensor's circuit is faulty, so the engine can't hear itself pinking. P0011 The camshaft timing on bank 1 is more advanced than the engine asked for. P0016 The crankshaft and camshaft position signals on bank 1 don't line up as they should. P0505 The idle speed control isn't working. P0507 The idle speed is higher than it should be.

Fuel and air

P0171 The engine is running lean (too much air, not enough fuel) on bank 1. P0174 The engine is running lean (too much air, not enough fuel) on bank 2. P0172 Bank 1 is running rich: too much fuel or too little air. P0101 The mass airflow (MAF) sensor's reading is out of the expected range for how the engine's running. P0102 The airflow sensor reads lower than the engine can be breathing. P0087 Fuel pressure in the rail is lower than the engine is asking for. P0093 The fuel system has detected what looks like a large fuel leak.

Emissions and exhaust

P0420 The catalytic converter on bank 1 isn't reducing emissions as efficiently as it should. P0430 The catalytic converter on bank 2 isn't cleaning the exhaust as well as it should. P0130 Circuit fault on the upstream (pre-cat) oxygen sensor, bank 1 sensor 1. It's the one that controls fuelling. P0133 The upstream O2 sensor (bank 1) is responding too slowly when switching between rich and lean. P0135 The heater circuit inside the upstream O2 sensor (bank 1) has failed. The sensor takes too long to start working after a cold start. P0138 The downstream (post-cat) O2 sensor on bank 1 is reading a constantly high voltage. Either a circuit fault or a genuinely rich-running engine. P0141 The heater circuit inside the downstream (post-cat) O2 sensor on bank 1 has failed. P0401 The EGR system isn't flowing enough exhaust gas back into the intake when commanded. P0402 Too much exhaust gas is recirculating through the EGR valve.

Fuel vapour (EVAP)

P0455 The EVAP system has a large leak. Most famously, a loose or missing fuel cap. P0442 The EVAP system (fuel vapour containment) has detected a small leak. Pinhole-sized, not dripping fuel. P0456 A very small leak in the fuel vapour system. P0440 A fault in the fuel vapour system, the one that stops petrol fumes escaping the tank. P0446 The EVAP vent valve or its circuit isn't working properly. It's a control fault, not a vapour leak.

Diesel (DPF, turbo, glow plugs)

P2463 Too much soot has built up in the diesel particulate filter. P246C The diesel particulate filter is restricted, and the car has limited engine power because of it. P2002 The diesel particulate filter on bank 1 isn't trapping soot as well as it should. P0299 The turbo isn't making the boost it's being asked for. P0234 The turbo is making more boost than it's being asked for. P0380 A glow plug circuit is faulty. These are the diesel's cold-start heaters. P0670 The glow plug control module's circuit is faulty.

Sensors and electrics

P0128 The engine is taking too long to reach normal operating temperature (coolant thermostat below regulating temp). P0118 The coolant temperature sensor signal is stuck high. The ECU sees an impossibly cold reading, pointing to a wiring or sensor fault. P0113 The intake air temperature sensor is reading implausibly cold. That's an open circuit or wiring fault, not actual cold air. P0335 The crankshaft position sensor's signal is faulty: the engine's basic sense of where the pistons are. P0340 The camshaft position sensor's signal is faulty: the engine's sense of where the valves are. P0562 The electrical system voltage is low.

Gearbox

P0700 The gearbox computer has stored a fault of its own.

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