Fault-code families
Fault codes grouped by system, so you can see a whole family at once — the shared causes, the parts involved, and every related code in one place.
💨 EVAP System Leak Codes
EVAP codes mean the evaporative emissions system — which captures fuel vapours from the tank and stops them escaping to the air — has a leak or a faulty valve. It is the single most common family of check-engine faults. The good news: many EVAP codes are caused by something as cheap as a loose gas cap.
🔥 Engine Misfire Codes
Misfire codes mean one or more cylinders are not burning fuel properly. P0300 is a random/multiple misfire; P0301–P0304 each point to a specific cylinder. A misfire causes rough running and, if the light is flashing, can quickly destroy the catalytic converter — so it is one to fix promptly.
🌬️ Lean Fuel Mixture Codes
A "lean" code means the engine is getting too much air or too little fuel, so the computer adds fuel to compensate and eventually sets a code. P0171/P0174 are the classic lean codes. Lean running causes hesitation, rough idle and, over time, misfires — and the cause is often an unmetered air (vacuum) leak.
🛡️ Catalytic Converter Codes
Catalyst codes mean the catalytic converter is no longer cleaning the exhaust efficiently enough. P0420 is Bank 1, P0430 is Bank 2. The converter is one of the most expensive emissions parts, but the code is not always the converter itself — a worn rear oxygen sensor or an exhaust leak can trigger it too.
🧪 Oxygen (O2) Sensor Codes
Oxygen-sensor codes mean one of the O2 (lambda) sensors in the exhaust is reading slowly, wrongly, or not at all. These sensors let the computer fine-tune the mixture and monitor the catalytic converter, so a bad one hurts economy and emissions. The code identifies the exact sensor by bank and position.
♻️ EGR System Codes
EGR codes mean the exhaust gas recirculation system — which routes a little exhaust back into the intake to cut emissions and knock — is flowing too much, too little, or has a stuck valve. The usual culprit is carbon build-up clogging the valve or its passages, which often cleans up rather than needing replacement.
⏱️ Engine Timing & Cam/Crank Sensor Codes
These codes involve engine timing — the precise synchronisation of the crankshaft and camshaft. They come from the camshaft and crankshaft position sensors, or from the variable valve timing (VVT) system that advances and retards the cams. A timing fault causes hard starting, stalling, rough running and lost power.
🌡️ Coolant Temperature Codes
These codes relate to engine temperature — either the coolant temperature sensor (ECT) reading wrongly, or the engine not reaching its proper operating temperature. The usual culprits are a stuck-open thermostat or a faulty sensor. They affect cold-start fuelling, fuel economy and the temperature gauge.
🌀 Air Intake & MAF/MAP Sensor Codes
These codes come from the sensors that measure the air entering the engine — the mass air flow (MAF) sensor, the manifold pressure (MAP) sensor, and the intake air temperature (IAT) sensor. A dirty or faulty air sensor makes the computer mis-fuel the engine, causing hesitation, rough idle, poor economy and stalling.