When clearing codes is right, what clearing erases, how to check readiness monitors and how to reset the service interval.
Clearing fault codes with a scan tool takes seconds, but doing it before a repair only hides the fault and resets the emissions monitors. Learn when it is right to clear codes, how to do it step by step, how to check that monitors are ready and how to reset the service interval.
The clear-codes command (Mode 04 in OBD-II) does not only erase codes: it also erases freeze-frames and returns the readiness monitors to "incomplete". Until the car meets each monitor's test conditions, the ECU cannot confirm that the emissions systems work. Resetting the service interval is a separate operation that simply restarts the count to the next service.
Original editorial schematic, illustrative only — not a real scan-tool screenshot and not a substitute for engine-specific procedures.
Read, save, repair, then clear. After a few days of driving, check that the monitors are complete and that no code has returned as pending.
Warning: A red oil pressure, brake or temperature warning means stopping safely, not clearing a code. Continuing can destroy the engine or endanger people.
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2. What actually gets cleared| Item | After clearing | What it means for you |
|---|---|---|
| Stored and pending codes | Erased | Save them first for the repair history |
| Freeze-frame | Erased | You lose the conditions in which the fault set |
| Readiness monitors | Back to "incomplete" | You must drive until they complete again |
| Adaptations and learned values | Usually kept | Disconnecting the battery can erase them, which is why it is not a way to clear codes |
| Service interval | Unchanged | Reset separately |
Connect the scan tool with the ignition on and read stored, pending and permanent codes plus freeze-frames. Save the report or take screenshots.
Diagnose and repair the indicated fault. Look the code up in the catalogue for symptoms, causes and checks.
Ignition on, engine off (or as the app requires), choose the clear-codes function and confirm. Do not disconnect the battery to clear codes: you lose adaptations, radio codes and, on some cars, the battery registration.
Read the codes again: codes that return immediately point to a fault that is still present, usually electrical. The warning light should go out after starting.
Drive in varied conditions over the following days: cold start, idle, steady extra-urban cruising, deceleration without braking. Each monitor has its own conditions; some complete within a few dozen kilometres, others after several days.
Warning: Do driving tests only in normal traffic, within legal limits. Do not watch the scan tool while driving.
Read readiness status in the scan tool. Every applicable monitor should be "complete", with no pending codes. Only then is the repair confirmed for emissions.
After a service, reset the service indicator from the scan tool or through the dashboard procedure in the manual (usually button combinations with the ignition on). The procedure differs by make and model.
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Codes and freeze-frames saved Cause diagnosed and repaired Codes cleared with the scan tool, not by disconnecting the battery Codes re-read right after clearing Varied driving to complete the monitors Monitors confirmed complete Service interval reset after the serviceYes, if the fault is still present. Some codes return immediately, others after the ECU repeats its test in the right conditions.
How long until the monitors complete?From a few dozen kilometres to several days of normal use, depending on the monitor and the car. The catalyst and evaporative monitors are usually the last.
Can I reset the service without a scan tool?On many cars yes, with dashboard button combinations described in the manual. Others need a compatible scan tool.
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