Cummins ISX CM870 (2003-06) Fault Code: 2377 PID: S33 SPN: 647 FMI: 3/3 Fan Control Circuit – Voltage Above Normal or Shorted to High Source. Open circuit or high voltage detected at the fan control circuit.

The ECM monitors the voltage level on this circuit. When the ECM commands the fan signal low, it expects the voltage level to be approximately 0 volts. If it detects a higher voltage, this fault is recorded. This usually indicates an open circuit or short circuit to a voltage source in the harness.

Once the fault code becomes active, the fan must be cycled to clear the code. This can be done by cycling the manual fan switch, if equipped, or by operating the engine and satisfying the fan engagement requirements.

If Fault Code 2377 is still active after completing the following troubleshooting steps, consult the OEM service literature for procedures to check the fan clutch device for an open circuit or short circuit to ground.

Possible repairs:

1)Replace the fan control solenoid.
2)Repair or replace the OEM harness.
3)Replace the ECM.