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Cummins ISX CM870 (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 relay circuit when commanded ON.

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.