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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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Lean / fuel-trim family

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. P0101 The mass airflow (MAF) sensor's reading is out of the expected range for how the engine's running.

Misfire

P0300 The engine is misfiring on multiple or random cylinders.

Oxygen (O2) sensor family

P0130 Circuit fault on the upstream (pre-cat) oxygen sensor, bank 1 sensor 1 — 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. P0141 The heater circuit inside the downstream (post-cat) O2 sensor on bank 1 has failed. 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.

Temperature sensors

P0128 The engine is taking too long to reach normal operating temperature (coolant thermostat below regulating temp). P0113 The intake air temperature sensor is reading implausibly cold — an open circuit or wiring fault, not actual cold air. P0118 The coolant temperature sensor signal is stuck high — the ECU sees an impossibly cold reading, pointing to a wiring or sensor fault.

EGR

P0401 The EGR system isn't flowing enough exhaust gas back into the intake when commanded.

EVAP (fuel vapour) family

P0442 The EVAP system (fuel vapour containment) has detected a small leak — pinhole-sized, not dripping fuel. P0446 The EVAP vent valve or its circuit isn't working properly — a control fault, not a vapour leak. P0455 The EVAP system has a large leak — most famously a loose or missing fuel cap.

Catalyst

P0420 The catalytic converter on bank 1 isn't reducing emissions as efficiently as it should.

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