Cummins ISB CM850 (2003-06) Fault Code: 1665 PID: SPN: 3241 FMI: 4 Exhaust Gas Temperature 1 Circuit ? Voltage Below Normal, or Shorted to Low Source. Low signal voltage detected at the catalyst inlet temperature sensor circuit.

The catalyst inlet temperature sensor shares return wires in the engine harness with other sensors. A shorted return can cause multiple fault codes to be active.

Possible causes:

1)Signal shorted to ground in the harness
2)Signal shorted to return or ground in the sensor

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 exhaust gas temperature #1 sensor.
2)The removal and installation of the connector may correct the fault.
3)Repair or replace the engine harness.
4)Replace the ECM.