Codes and warning lights
Why does the check engine light come on and then go off by itself?
Because the ECU turns the light off when the fault stops showing up for several drive cycles in a row, even though it keeps the code in memory. It is usually an intermittent fault: a loose connector, a sensor that fails when cold or hot, a loose gas cap or a lean mixture under certain conditions.
Reviewed by Daniel Morales, founder of Reprorace SL, on
Which intermittent faults are the most common?
Misfires that only happen when cold or under load, small fuel vapor leaks, slow oxygen sensors, corroded connectors and battery voltage dips when starting.
The fault often depends on the weather, the temperature or the kind of trip, which is why it comes and goes.
Can I forget about it once it is off?
Not entirely. The code stays stored for a while and, with the freeze frame data, tells you the conditions it happened in, which helps a lot in finding the cause.
If the fault gets worse, the light comes back and may stay on; if it is a misfire, it may flash and damage the catalytic converter.
What should I do next time it comes on?
Note when it happens: cold, uphill, after refueling or in the rain. Check that the gas cap closes properly and that there is no fuel smell.
Read the code while it is on or right after, without clearing it, and take it to the shop along with what you noticed.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
Related
Related trouble codes
- P0300Random or multiple cylinder misfire detected
- P0455EVAP system: large leak detected
- P0171Mixture too lean (bank 1)
- P0133O2 sensor (bank 1, sensor 1), circuit: slow response
Related symptoms
- Engine warning light on and steady: what it means and what to do
- Flashing engine warning light: what it means and what to do
Learn more
See what your car is saying
The ECO-LITE stays in the port and keeps the codes even after the light has gone off, with the data from the moment of the fault, so your shop can see them with your permission.
Presale price: €129 · US$129
Frequently asked questions
Is it the same if it flashes and then goes off?
No: flashing means serious misfires at that moment. Ease off and have it checked soon.
Could it be the battery?
Low voltage when starting can leave codes in several modules; if it matches slow starts, check the battery.
How long does the code stay stored?
It depends on the manufacturer; in standard OBD, about 40 warm-up cycles without the fault coming back.