A yellow or orange check engine light has come on and you are not sure whether you can keep driving? It is one of the most common questions drivers search for. This guide explains what the light means, when it is serious and how to find the cause quickly.
What does the check engine light mean?
The engine-shaped warning light is part of the OBD (On-Board Diagnostics) system fitted to every modern car. The engine control unit constantly monitors sensors – the fuel-air mixture, ignition, exhaust gases, temperature and catalytic converter efficiency. When it detects a deviation, it stores a fault code and switches on the light.
So the light does not tell you what is wrong – only that the control unit has detected a problem. The cause can be trivial or serious.
Steady or flashing? A big difference
- Steady light – the fault is not urgent, but have it checked within a few days. You can usually keep driving, but watch for changes in how the engine runs.
- Flashing light – often means a misfire (one cylinder is not firing properly). Unburnt fuel can damage the expensive catalytic converter in a short time. Reduce load, avoid hard acceleration and get the car checked as soon as possible.
If the light comes on together with a red warning light (oil pressure, coolant temperature), stop safely and switch off the engine.
The most common causes
1. A loose fuel cap
Petrol cars check the fuel vapour system for leaks. A loose cap can trigger the light. Tighten it until it clicks – the light may go off by itself after a few drives.
2. Oxygen (lambda) sensor
It measures oxygen in the exhaust and helps regulate the mixture. A faulty sensor increases fuel consumption and emissions.
3. Spark plugs and ignition coils
Worn plugs or a failed coil cause misfires, a shaking engine and a flashing light.
4. Mass air flow (MAF) sensor
A dirty or faulty sensor misreads the amount of air entering the engine, affecting power and fuel economy.
5. EGR valve and diesel particulate filter (DPF)
Diesel cars used mostly for short city trips often clog them. The light can be the first sign that the filter needs regeneration or cleaning.
6. Catalytic converter
If the converter is no longer efficient, the control unit detects it through its sensors. Failure is often the result of a long-ignored misfire.
Finding the cause: a computer diagnostic scan
The fastest way to the cause is a computer diagnostic scan. A technician connects a diagnostic tool to the OBD port under the dashboard and reads stored fault codes and live sensor data. A good garage will not just clear the code but find out why it appeared.
Cheap OBD readers for your phone can read basic codes, but they do not replace an experienced mechanic who can interpret the code and find the real cause. Read more in our car diagnostics guide.
Can I just clear the light?
You can, but it will come back if the cause has not been fixed – and clearing it erases data that would help the mechanic. A lit engine light can also mean failing your roadworthiness inspection.
What to do when the light comes on
- Check whether the light is steady or flashing.
- Check whether any red warning lights are on.
- Tighten the fuel cap.
- Watch for shaking, loss of power or unusual smells.
- Book a diagnostic scan as soon as possible.
Find a garage for diagnostics
Kardet lists garages that offer computer diagnostics. Send a free request with your make, model and when the light came on, and local garages will send you their offers.