Cummins ISM CM875 (2003-06) Fault Code: 339 PID: S087 SPN: 1267 FMI: 4/4 Idle Shutdown Vehicle Accessories Relay Driver Circuit – Voltage Below Normal, or Shorted to Low Source. Low voltage detected at the idle shutdown vehicle accessory or ignition bus r

The ECM monitors the voltage level on this circuit. This fault code is logged if it detects a low voltage when the relay signal is commanded on. This fault code can be caused by a short circuit to ground in the idle shutdown relay, OEM harness, or connectors.

Possible repair:

1)Replace the idle shutdown relay.
2)Repair or replace the OEM harness.