Depends on the type of fault code as to how long it will take to clear. The light might go out sooner. A code won't reset for 40 warm-up cycles without being reset. Disconnecting the battery will reset the whole car and the light. Depending on the car that could be a real bad thing to do.

Upon reseting with a code reader or scan tool most emissions stations will then require the monitors to run. This will require proper application of a drive cycle. In reality depending on model the monitors can take some time to run.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician