The PCM monitors ignition voltage on circuit 439 when powered up. The purpose of this diagnostic trouble code is to detect an unstable ignition voltage to the PCM.
This DTC sets from an internal PCM failure. The TCM sees battery voltage out of range high or low.
Check for DTC P0562 and P0563. If no DTCs are present, replace the PCM.
If DTC P1635 is also set, diagnose DTC P1635 first.