The right way is to fix the problem; the light then goes out by itself once the car confirms the repair over a few drives. You can also clear the code with an OBD2 adapter, but that only turns the light off. If the problem is still there it comes back, and clearing resets the emissions monitors, so the car can fail an inspection until it has been driven for a while.
After a repair, the computer runs the test that failed again. Once it passes on enough trips, often about three, the light goes out on its own. Shops usually clear the code after a repair anyway, so you can see right away whether it returns.
An OBD2 adapter can send the clear command. It erases the stored codes, the snapshot of what the engine was doing when the fault happened, and the readiness monitors. Good reasons to do it: a repair is done, or you want to see whether a fault comes back.
In our app, Clear codes sits behind a plain warning first, because it does not fix anything.
On many cars this clears the codes too, but it also resets the radio, the clock and the engine's learned settings, and some cars then need a relearn procedure. We do not recommend it as a way to turn off a light.
If the fault is still there, the car finds it again. Some faults return within minutes, others after the next cold start or a few days.
Cars from model year 2010 on also keep permanent codes that no adapter can erase. They clear only after the car confirms the repair itself. That is one way an inspection station can tell a code was cleared recently.
Clearing sets every readiness monitor back to not ready. Many emissions inspections turn away a car with too many monitors not ready, so a freshly cleared car can fail even with no light on. Most monitors finish after a few days of mixed city and highway driving. See our guide to the check engine light and inspections.
This is also why our Used-Car Check shows how long ago codes were cleared: a seller can clear them right before your test drive.
On many cars it does, but it also resets the radio, clock and the engine's learned settings, and it does not fix anything. If the fault is still there, the light comes back.
Yes, if the problem is fixed or was temporary. The car rechecks the fault on later trips, and after it passes a few times in a row the light usually goes out.
It depends on the car and the monitor. A few days of normal driving with a mix of city and highway speeds and at least one cold start usually sets most of them. The fuel vapor monitor can take longer.
Estimates only. A mechanic should confirm. If the car shows a hot temperature gauge, an oil pressure warning, smoke or a burning smell, stop as soon as it is safe.