lookup code 691 : Compressor Inlet Temperature Sensor Circuit

Fault code 691 in trucks typically refers to an issue with the Turbocharger #1 Compressor Inlet Temperature Sensor Circuit. This fault code indicates that the voltage is above normal or shorted to a high source. Here are some possible causes and effects of this fault code:

Possible Causes:

- Open return circuit in the harness, connectors, or sensor.

- Open signal circuit or short circuit to a voltage source.

- Failed sensor.

Effects:

- Engine power derate.

Troubleshooting Steps:

1. Check for multiple fault codes before troubleshooting Fault Code 691.

2. Inspect the turbocharger compressor inlet air temperature sensor and connector pins for any damage or dirt.

3. Check the sensor resistance and verify its condition.

4. Inspect the ECM (Engine Control Module) and engine harness connector pins.

5. Check for an open circuit in the engine harness.

If you need more detailed information or specific troubleshooting steps, you can refer to the [detailed guide](http://ww2.justanswer.com/uploads/JCTech012/2010-02-04_201754_FC691.pdf) or consult a professional mechanic.