Cummins ISX CM870 (2003-06) Fault Code: 551 (ISS) PID: S230 SPN: 558 FMI: 4/4 Accelerator Pedal Idle Validation Circuit – data incorrect. No voltage detected simultaneously on both idle validation and off-idle validation switches.

ISS Goes to 70-fc 551iss

Possible causes:

1)A short circuit to ground in the harness, uncalibrated accelerator pedal assembly, or miswired idle validation switch.
2)When installing a new accelerator pedal assembly, it must be calibrated before operating the engine. To calibrate, turn the keyswitch to the ON position and fully depress and release the pedal three times.

Possible repairs:

1)Replace the ISS.
2)Replace the ECM.
3)Repair or replace the OEM harness.