Cummins ISB CM850 (2003-06) Fault Code: 2556 PID: SID 70 SPN: 729 FMI: 4 Intake Air Heater #1 Circuit – Voltage Below Normal or Shorted to Low Source. Low voltage detected at the intake air heater signal circuit.
Possible causes:
1)A shorted intake air heater relay
2)Intake air heater relay supply wire shorted to ground.
Possible repairs:
1)Replace the damaged intake air heater relay.
2)Repair or replace the OEM harness.
3)The removal and installation of the connector may correct the fault.
4)Replace the ECM.