Cummins ISB CM850 (2003-06) Fault Code: 551 (NISS) PID: SID 230 SPN: 556 FMI: 4 Accelerator Pedal or Lever Idle Validation Circuit – Voltage Below Normal or Shorted to Low Source. No voltage detected simultaneously on both the idle validation off-idle and on-idle circuits.

1)This fault code is usually caused by a short circuit to ground in the harness, an uncalibrated accelerator pedal or lever assembly, or a miswired idle validation switch.
2)When installing a new accelerator pedal or lever 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 or lever three times.

Possible repairs:

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