Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.