When the power to the PCM is removed, depending on how long the power is removed would dicate how much of what is in PCM memory is lost or removed.

Some people think that by disconnecting the main car battery for a few minutes will clear everything out, when in fact it can takes several hours for everything to be drained due to no power to the PCM.

If the battery power to the PCM is removed for a period of several hours, all trouble codes, freeze frame data, fail records, last test failed messages and the I/M Readiness status are lost.

Note: This is not a recommended method to clear the memory of the PCM to clear codes because this action will cause the Diagnostic Executive (PCM) to lose all of its diagnostic test records.

On G.M cars, the PCM clears a code when it determines that FORTY warmup cycles have occurred without a fault since the MIL was turned off.