Clearing fault codes before an emissions test
Clearing turns the check engine light off, but it doesn’t repair anything, and it resets the self-tests that inspections reading OBD check. Repair first, clear, then drive.
Updated
What clearing does
- Erases the stored and pending fault codes, and the check engine light goes off.
- Resets emissions readiness: the self-tests that inspections reading OBD, such as the Dutch APK and the German emissions test (AU), check.
- Erases the freeze frame, the readings saved when the fault was set. Note it first if a mechanic needs it.
- Leaves permanent codes listed, on cars that have them, until the car re-tests the repair.
If the cause is still there, the light comes back.
Emissions readiness
The car tests its emission systems itself while you drive: catalytic converter, oxygen sensors, EGR, fuel vapour system and others, depending on the engine. Each self-test is either complete or not complete.
After clearing, they all start again. They need a few days of normal driving, often 50–200 km, and some only run under certain conditions. Read My Car shows them under Vehicle, emissions readiness.
Before an inspection
- Have the cause of the fault repaired.
- Park the car and switch the engine off. Keep the ignition on.
- Clear the codes. Read My Car then reads them again to check whether they came back.
- Drive normally for a few days, often 50–200 km.
- Check emissions readiness and the fault codes before the inspection.
The Dutch APK
For cars first used after 31 December 2005 that run on petrol only (not LPG or CNG) or on diesel, the APK inspector reads the engine computer over OBD. If the check engine light works, there are no stored fault codes (P codes) and the self-tests are complete, the OBD emissions check is passed. Otherwise the usual exhaust gas test follows. Since 1 January 2023, diesels with a particulate filter get a particle count instead.
Other countries’ inspections differ in what they test. Check the rules where your car is inspected.
Clearing doesn’t hide a fault
Where an inspection reads OBD, clearing just before it doesn’t hide a fault: the reset self-tests show that codes were cleared. Clearing to mislead an inspector or a buyer isn’t a fix, and can be illegal.
Questions
Can I clear the codes right before the inspection?
It doesn’t help. The self-tests are reset as well, and an inspection that reads OBD sees that they aren’t complete. Repair, clear, and drive normally for a few days first.
Why is a code still listed after clearing?
It’s probably a permanent code. Those can’t be cleared: the car removes them itself once it has re-tested the repair.
Can I clear codes with the engine running?
Clear with the car parked, the engine off and the ignition on. Read My Car waits until the engine is off on a real car.