Codes and warning lights
How long does the check engine light take to go off after the repair?
On most cars, the check engine light goes off by itself after about three drive cycles in a row in which the ECU tests the system again and does not see the fault. A shop or a scan tool can clear it right away; if it comes back on, the fault was not fixed.
Reviewed by Daniel Morales, founder of Reprorace SL, on
What is a drive cycle?
It is a cold start, a trip in which the engine warms up and runs in the conditions each test needs, and the shutdown. Some tests, such as the catalytic converter or the fuel vapor leak test, only run at a given speed, temperature or fuel level.
That is why some lights go off in two days and others take more than a week if you only make short trips.
Should I clear the code after the repair?
Yes, if you do it after fixing the fault and writing the code down: that way you soon see whether it returns. Keep in mind that clearing also resets the readiness monitors and the freeze frame data.
Do not clear without repairing to pass an inspection or sell the car: the fault returns and the incomplete monitors give it away.
What if the light is still on after several days?
Read the codes again. If the same one shows up, that was not the cause or another part is involved; if it is a different one, it may be a second fault that the first one was hiding.
If the light flashes or the car runs worse, do not wait for it to go off: stop and have it checked.
General guidance only: every manufacturer and model has its own details. Always follow your vehicle's manual and, if in doubt, ask a workshop.
See what your car is saying
The ECO-LITE shows you in your account whether the code is still active, has become a history code or returns, and how the monitors are doing, without waiting for the light to go off.
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Frequently asked questions
Does the light go off if I disconnect the battery?
On many cars yes, but the monitors and the fault data are erased too; if the fault is still there, it comes back.
Is the code erased from memory when the light goes off?
Not always: in standard OBD it stays as a history code for about 40 warm-up cycles.
Do ABS or airbag lights go off by themselves?
Some do at the next start if the fault is gone; others need to be cleared with a diagnostic tool.