Warning lights
Engine warning light on and steady: what it means and what to do
A steady engine warning light means the engine control unit has detected a fault, almost always related to emissions, and stored a code. It rarely means you must stop, but read the code and have it checked soon, before the fault damages other parts.
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Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. If the car drives normally, with no jerking, noises or other lights, you can drive calmly to the garage in the next few days. If the light starts flashing, you lose power or the temperature rises, stop as soon as it is safe.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Fuel cap or a leak in the vapor system (EVAP): a fuel cap left loose is enough to light it on many gasoline cars.
- Oxygen sensor: an aging sensor or a failed sensor heater gives false mixture readings.
- Inefficient catalytic converter: often the result of an earlier fault, such as misfires or oil burning.
- Air-flow meter or air leaks: a dirty air-flow meter or a cracked hose upsets the mixture.
- EGR valve or thermostat: the EGR valve clogs with use, especially on diesels; a thermostat stuck open keeps the engine from warming up.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: What does a pending, stored or permanent code mean? Why does the check engine light come on and then go off by itself? How long does the check engine light take to go off after the repair?
Car system: Exhaust and emissions
What you can check yourself
Make sure the fuel cap is closed until it clicks. If it was loose, the light may take several journeys to go out.
Notice whether the car behaves differently: jerking, higher consumption, smoke or an uneven idle.
Read the fault code and save it before clearing anything.
If the car is due its MOT or periodic inspection, have it checked first: the light is part of the test.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
- P0420Catalyst system (bank 1): efficiency below threshold
- P0171Mixture too lean (bank 1)
- P0457EVAP system: leak detected (fuel cap loose or missing)
- P0455EVAP system: large leak detected
- P0135O2 sensor heater (bank 1, sensor 1), circuit: malfunction
- P0401EGR A flow: insufficient flow detected
- P0128Coolant thermostat: coolant temperature below the thermostat regulating temperature
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: coolant temperature, short- and long-term fuel trims, oxygen sensors and air flow.
Frequently asked questions
Will the car pass the MOT with the engine light on?
Usually not: the periodic inspection checks the engine warning light, and a light that stays on is normally a fail or a defect. Fix the fault before the test.
Can I switch the light off by clearing the code?
It goes out, but if the fault is still there it comes back. Clearing also erases the freeze-frame data that help the garage and resets the readiness monitors.
Can the light go out by itself?
Yes. If the control unit no longer sees the fault over several drive cycles it switches the light off, but the code stays in the history for a while.
Is it the same as the service light?
No. The service light (a wrench or the word SERVICE) only reminds you of maintenance. The engine light is shaped like an engine and shows in amber.
Keep reading
- Exhaust and emissions: how they work and what fails
- Flashing engine warning light: what it means and what to do
- Car uses more fuel than normal: causes and what to check
- Car loses power: what limp mode is
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means