Circuit DescriptionThe 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 electronic control module (ECM) supplies 5-VDC 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 LocationSome engines use a combination ambient air pressure/temperature 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 your engine has.

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

Conditions for Setting the Fault CodesThe ECM detects that the compressor intake temperature signal voltage is greater than 4.96-VDC for more than 10 second.

Action Taken When the Fault Code is ActiveThe 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 CodeTo validate the repair, start the engine and let it idle for 1 minute.The fault code status displayed by the INSITE? electronic service tool will change to INACTIVE immediately after the diagnostic runs and passes.The ECM will turn off the amber CHECK ENGINE light immediately after the diagnostic runs and passes.For On Board Diagnositc (OBD) engines, the ECM will turn off the MIL after 3 consecutive trips where the diagnostic runs and passes.The ?Reset All Faults? command in the INSITE? electronic service tool can be used to clear active and inactive faults, as well as extinguish the MIL for OBD applications

Shop TalkThe 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 and the turbocharger compressor intake temperature sensor can be used depending on the turbocharger arrangement.

Possible causes of this fault code:

Open return circuit in the harness, connectors, or sensorOpen signal circuit or short circuit to a voltage sourceDamaged sensor.

Possible Cause:1) ECM failure2) Engine Harness3) OEM harness4) ECM calibration