PCM Long Term Memory and Engine Off Diagnostics

Several ignition positive voltage circuits supply battery power to the PCM even when the ignition switch is turned OFF. The continuous battery power enables the PCM to support a long term memory. The long term memory provides storage of diagnostic trouble codes, diagnostic test status, and the aforementioned "learned values" that improve driveability.

Another use of continuous battery power is the PCM can run certain system diagnostics with the engine off. One of these diagnostics is the engine off natural vacuum (EONV) EVAP system leak test. With the engine OFF, the PCM can turn on the main relay and provide power to operate the EVAP solenoids while monitoring the fuel tank pressure. During the running of the EONV diagnostic routine, the PCM can be fully powered up for as long as 30 minutes.