Clearing a fault code: when it helps and when it is a bad idea

What clearing actually does

When you confirm clearing in DiagAuto AI, the app sends the standard OBD2 “clear codes” command to the engine control unit; just before that, it offers to save the report in the history. The control unit then:

• deletes the confirmed and pending codes;
• deletes the data at the time of the fault (the “freeze frame”: the values recorded when the fault appeared);
• turns off the check engine light;
• sets the emissions checks (the “readiness monitors”) back to “not ready”: the control unit has to run its self-tests again over several drive cycles.

What clearing does not do: fix the cause, clear “permanent” codes (found on recent cars, they only go away once the control unit has itself confirmed that the fault is gone), or touch the other control units or any settings.

When it helps

1. After a repair, to check that it worked.
2. To isolate a fault: after swapping a coil, cleaning a mass air flow sensor, tightening a fuel cap (see P0455).
3. After a known one-off event: a tank of wrong fuel that has been dealt with, a disconnected battery, a sensor unplugged by mistake.

When it is a bad idea

1. Before you have noted the code and the data at the time of the fault: you lose information your mechanic (or the AI) would have used. Take a screenshot, or save the report the app offers just before clearing (PDF export is a Premium feature).
2. Before the roadworthiness test (contrôle technique in France, similar to the UK MOT): the emissions checks go back to “not ready”, the light comes back if the fault is still there, and on some recent cars the inspector can also query the diagnostic port. You risk a re-test without having fixed anything. Read Roadworthiness test: check your codes beforehand.
3. To hide a fault from a buyer: that is deception, and a buyer with a scanner can spot it (emissions checks “not ready”, permanent codes). See Buying a used car: scan it before you sign.
4. When the light is flashing: do not clear the code just to keep driving. Here is why.

How to do it properly

1. Read all the codes and the data at the time of the fault. Save the report (the app offers to before clearing; PDF export with Premium) or take a screenshot.
2. Carry out the repair or the test.
3. Clear the codes with the engine off and the ignition on, after confirming (DiagAuto AI does no reprogramming).
4. Drive a full cycle: cold start, 10 minutes in town, 15 minutes on the open road, idle, switch off. Repeat 2 or 3 times.
5. Read again: no code and emissions checks “ready” = fixed. A code that returns = the cause is still active, but you now know the fault is reproducible.