The engine warning light, often called the check engine light or engine management light, can appear for anything from a loose fuel cap to a serious misfire. The light itself doesn't tell you much, but the fault code stored behind it does. Here is how to read those codes, what some of the most common ones usually mean, and which genuine parts are often behind them.
Steady or flashing?
- A steady light means the engine control unit has found a fault. It is usually safe to drive carefully to a garage or home, but get it checked soon.
- A flashing light usually means a severe misfire that could damage the catalytic converter. Reduce speed, avoid heavy acceleration and get the car checked as soon as possible.
If the car also loses power, overheats or shows a red warning, stop safely and seek help.
Reading the code
Every car sold in the UK since the early 2000s has an OBD socket, usually under the dashboard near the steering column. An inexpensive code reader, or a garage, can read the stored code. Engine codes start with a P for powertrain, followed by four characters. Codes starting P0 are generic and mean the same across makes, while codes starting P1 are often manufacturer-specific.
Remember that a code tells you which system has noticed a problem, not always which part has failed. Clearing the code without fixing the cause will just bring the light back.
Common codes and what they usually mean
Misfire codes: P0300 to P0304 and beyond
P0300 means random or multiple misfires. P0301, P0302 and so on point to a specific cylinder. Common causes include worn spark plugs, a failing ignition coil, a faulty injector or an air leak. Swapping a coil from the misfiring cylinder to another and seeing if the fault moves is a simple test. Our guide to choosing spark plugs explains why the right plug matters.
Lean mixture: P0171 and P0174
These mean the engine is running with too much air or too little fuel. Common causes are vacuum leaks from split hoses or intake gaskets, a dirty or faulty mass airflow sensor, or a weak fuel supply.
Mass airflow sensor: P0100 to P0104
The MAF sensor measures the air entering the engine. A faulty or contaminated sensor can cause hesitation, poor economy and flat spots. Check the air filter and intake for leaks before replacing the sensor. Our guide to engine and cabin air filters covers why the filter seal matters.
Oxygen (lambda) sensors: P0130 to P0167
Lambda sensors measure oxygen in the exhaust so the engine can fine-tune fuelling. Codes in this range can point to a slow or failed sensor, a wiring fault or a heater circuit fault. Sensor 1 is usually before the catalytic converter and sensor 2 after it.
Catalytic converter efficiency: P0420
This code means the catalytic converter isn't cleaning the exhaust as well as expected. It can mean the converter is worn, but a faulty downstream lambda sensor, an exhaust leak or an earlier misfire can cause the same code.
Diesel-specific codes
Diesels often log codes for the EGR valve, DPF pressure sensor, glow plugs or boost pressure. Our guide to DPF and EGR trouble explains these systems in more detail.
Make-specific points
- Volkswagen Group: many faults are stored with manufacturer-specific codes alongside the generic P-codes, and a dealer-level scan tool can give more detail.
- Ford: some engines are sensitive to vacuum leaks and intake sensor faults that show up as lean codes.
- Toyota: lambda sensors are often split into air-fuel ratio sensors before the catalyst and oxygen sensors after it, and they are not interchangeable.
- Hyundai and Kia: sensor part numbers often change between engine types and production years, so check carefully.
Why genuine sensors are worth it
Engine sensors send precise signals that the control unit relies on to calculate fuelling, ignition and emissions. A sensor with a slightly different response time, resistance or connector can cause the same fault to return, or create new ones. Genuine sensors are made to the exact specification the engine was calibrated with, which is why they are usually the most reliable fix.
Sensors can vary by engine code, emissions standard and production date. Use your VIN to confirm the part number. Our guides to using your VIN and reading exploded parts diagrams show how.
Shop genuine sensors and engine parts
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