CAT 3406E (Prefix 1MM or 2WS) Fault Code: 21 PID: SPN: 41 FMI: 4 8 Volt Supply voltage low

The Engine Control Module (ECM) detects the following conditions:

•The 8 volt supply is less than 7.5 VDC for more than one second.
•The ECM has been powered for more than three seconds.

Probable Causes:

1) ECM Connection/Harness
2) Accelerator Position Sensor
3) Accelerator Position Sensor Harness
4) ECM Failure