Cummins ISB4.5 CM2150 Fault Code: 691 PID: 351 SPN: 1172 FMI: 3 Turbocharger 1 Compressor Intake Temperature Sensor Circuit – Voltage Above Normal or Shorted to High Source.

Posted by PerDiezel Solutions on January 20, 2026

Circuit Description
The compressor intake air temperature sensor is a variable resistor sensor and is used to measure the temperature of the air entering the compressor intake of the turbocharger. The engine control module (ECM) supplies 5 volts to the compressor intake temperature signal circuit. The ECM monitors the change in voltage caused by changes in the resistance of the sensor to determine the air temperature.

Component Location
Some engines use a combination ambient air temperature/pressure sensor located in the OEM intake air plumbing rather than a turbocharger compressor intake temperature sensor. Refer to the engine wiring diagram to determine which sensor this application uses.

Conditions for Running the Diagnostics
This diagnostic runs continuously when the keyswitch is in the ON position.

Conditions for Setting the Fault Codes
The ECM detects that the compressor intake temperature signal voltage is greater than 4.8-VDC for more than 1 second.

Action Taken When the Fault Code is Active
-The ECM illuminates the amber CHECK ENGINE light and/or the malfunction indicator lamp (MIL) immediately when the diagnostic runs and fails.
-A default value for the compressor intake temperature reading is used.

Conditions for Clearing the Fault Code
-To validate the repair, start the engine and let it idle for 1 minute.
-The ECM will turn off the amber CHECK ENGINE light immediately after the diagnostic runs and passes.
-The ECM will turn off the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and passes.
-This fault code and malfunction indicator lamp (MIL) can also be cleared with INSITE™ electronic service tool.

Shop Talk
The turbocharger compressor intake air temperature sensor shares return wires in the engine harness with other sensors. An open return can cause multiple fault codes to be active. Before troubleshooting Fault Code 691, check for multiple fault codes. An extension harness between the main engine harness turbocharger compressor intake temperature sensor can be used, depending on the turbocharger arrangement.

Possible Causes
-Open return circuit in the harness, connectors, or sensor
-Open signal circuit or short circuit to a voltage source
-Damaged sensor.

Cummins ISB4.5 CM2150 Fault Code: 689 PID: 190 SPN: 190 FMI: 2 Engine Crankshaft Speed/Position – Data Erratic, Intermittent, or Incorrect.

Cummins ISB4.5 CM2150 Fault Code: 688 PID: SPN: 98 FMI: 0 Engine Oil Level – Data Valid but Above Normal Operating Range – Most Severe Level.

Cummins ISB4.5 CM2150 Fault Code: 687 PID: 103 SPN: 103 FMI: 1 Turbocharger 1 Speed – Data Valid but Below Normal Operating Range – Moderately Severe Level.

Cummins ISB4.5 CM2150 Fault Code: 686 PID: 103 SPN: 103 FMI: 2 Turbocharger 1 Speed – Data Erratic, Intermittent, or Incorrect.

Cummins ISB4.5 CM2150 Fault Code: 649 PID: SID 115 SPN: 1378 FMI: 11 Engine Oil Change Interval – Condition Exists.

Cummins ISB4.5 CM2150 Fault Code: 598 PID: 167 SPN: 167 FMI: 1 Electrical Charging System Voltage – Data Valid But Below Normal Operating Range – Most Severe Level.

WARNING!