Cummins ISB CM850 (2003-06) Fault Code: 1684 PID: SPN: 3363 FMI: 4 Catalyst Tank Heater Circuit ? Voltage Below Normal, or Shorted to Low Source. Low signal voltage detected at the catalyst tank heater relay sensor circuit.

Posted by PerDiezel Solutions on January 18, 2026

This fault code cannot detect any circuit failures on the heater side of the relay. Only circuit errors between the ECM and the relay can be detected by this fault code.

Possible causes:

1)A shorted catalyst tank heater relay
2)Catalyst tank heater signal wire shorted to ground.

On-Board Diagnostics (OBD) Information (Euro 4 Certified Engines):

1)The ECM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
2)The ECM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail. The MIL lamp and fault code can also be cleared using the INSITE? service tool.
3)The fault code will be cleared from memory after 40 consecutive drive cycles where the diagnostic runs and passes.

On-Board Diagnostics (OBD) Information (Euro 4 Stage 1+ Certified Engines):

1)The ECM illuminates the malfunction indicator lamp (MIL) when the diagnostic runs and fails.
2)An engine torque derate will be activated after 50 hours of engine operation with the fault code active.
3)The ECM turns OFF the malfunction indicator lamp (MIL) after 1 ignition cycle that the diagnostic runs and does not fail. The MIL lamp cannot be cleared using the INSITE? service tool.
4)The fault code will be cleared from memory after 400 days or 9600 hours of engine operation.

Possible repairs:

1)Replace the catalyst tank heater relay.
2)The removal and installation of the connector may correct the failure.
3)Repair or replace the engine harness.
4)Replace the ECM.

Cummins ISB CM850 (2003-06) Fault Code: 1683 PID: SPN: 3449 FMI: 3 Catalyst Tank Heater Circuit ? Voltage Above Normal, or Shorted to High Source. High signal voltage detected at the catalyst tank heater relay sensor circuit.

Cummins ISB CM850 (2003-06) Fault Code: 1682 PID: SPN: 3362 FMI: 11 Aftertreatment Diesel Exhaust Fluid Dosing Unit Input Lines – Condition Exists. An error has been detected by the catalyst dosing control unit.

Cummins ISB CM850 (2003-06) Fault Code: 1681 PID: SPN: 3361 FMI: 12 Catalyst Dosing Control Unit ? Bad Intelligent Device or Component. An internal error has been detected in the catalyst dosing control unit.

Cummins ISB CM850 (2003-06) Fault Code: 1679 PID: SPN: 3031 FMI: 2 Catalyst Tank Temperature ? Data Erratic, Intermittent, or Incorrect. The catalyst solution temperature sensor is not changing with engine operating conditions.

Cummins ISB CM850 (2003-06) Fault Code: 1678 PID: SPN: 3031 FMI: 3 Catalyst Tank Temperature ? Voltage Above Normal, or Shorted to High Source. High signal voltage or open circuit detected at the catalyst tank temperature sensor circuit

Cummins ISB CM850 (2003-06) Fault Code: 1677 PID: SPN: 3031 FMI: 4 Catalyst Tank Temperature ? Voltage Below Normal, or Shorted to Low Source. Low signal voltage detected at the catalyst solution temperature sensor circuit.

WARNING!