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Clear codes and pass inspection

Clearing codes turns off the light but also resets the readiness monitors. The car is not ready for an emissions test until enough driving has run those self-tests again.

Clearing also resets
readiness monitors and freeze frame
Cannot be cleared
permanent codes, until the system passes

Each OBD2 vehicle runs self-tests called readiness monitors. Some run all the time (misfire, fuel system, comprehensive components). Others run only under certain conditions, such as the catalyst, EVAP, oxygen sensor, oxygen sensor heater and EGR monitors.

Clearing codes, or disconnecting the battery, resets all monitors to not complete. Inspection programs typically allow only a small number of incomplete monitors, and the exact rule varies by state and model year. Driving through a mix of city and highway driving, with a cold start, usually sets them; the vehicle's service information describes the exact drive cycle.

Permanent codes cannot be erased with a scan tool. They clear themselves once the monitor has run again and passed, which is why the real repair always comes first.

Ready when: The readiness status shows every supported monitor as complete, or within your state's allowance, and no codes are stored or pending.

Common mistakes

  • Clearing codes the day before an inspection.
  • Disconnecting the battery to turn off the light.
  • Assuming the problem is fixed because the light stayed off for a day.

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