PACCAR MX (2004 Emissions) Fault Code: P0685 PCI ECU – Short Circuit to Ground on ECU (D420) Pin (B3)

PACCAR MX (2004 Emissions) Fault Code: P0685 PCI ECU – Short Circuit to Ground on ECU (D420) Pin (B3)

SET CONDITIONS:
The PCI ECU (D420) detects a short circuit to ground on pin (B03).

DIAGNOSTIC CONDITION:
This diagnostic runs when the ignition is keyed on and off.

CORRESPONDING OBD READINESS MONITOR:
None

RESET CONDITION:
This DTC changes to inactive when the fault is no longer detected.

REPAIR VERIFICATION:
1. Clear all DTCs
2. Test drive or operate the vehicle to meet the diagnostic conditions and set conditions.
3. Did the DTC come back?
– No, repair successful
– Yes, repair unsuccessful

POSSIBLE CAUSES:
Short circuit to ground in the circuit of ECU pin B3.

ADDITIONAL INFORMATION:
The PCI ECU (D420) activates and deactivates its own main power by energizing relay (3).

When the ignition is:
1. Switched on, power supply is supplied to ECU pin B1. The ECU activates the relay (3) by switching ECU pin B3 to ground, and power supply is available on ECU pins B60/B61/B62.

2. Switched off, power supply is removed from ECU pin B1. The ECU performs different tasks, like storing data, and subsequently deactivates the relay (3) by removing the ground from ECU pin B3. The power supply is then removed from ECU pins B60/B61/B62.

Depending on the OEM electrical system architecture, it is possible that the PCI ECU remains powered on when the ignition is keyed off with this DTC active.