U0691: Lost Communication With Exhaust Pressure Sensor A

What it means

The vehicle bus reports loss of communication with smart exhaust pressure sensor A (typically pre-DPF or pre-turbine). Pressure-based diagnostics switch to a model-based estimate.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

Exhaust pressure sensor A is a smart sensor that typically measures pressure before the DPF or turbine and sends it over the vehicle bus. This code sets when sensor A's messages are lost, so pressure-based exhaust diagnostics have to use a model-based estimate.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Save freeze-frame data and scan all modules for communication codes to determine when pre-DPF or pre-turbine sensor A went offline and whether other bus devices also dropped out.
2. Inspect sensor A's connector and harness at its exhaust-side location for loose pins, corrosion, heat damage, or chafing.
3. Check the sensor module's power and ground according to the manufacturer's procedure, since loss of either can stop communication.
4. Inspect the bus wiring and splices to sensor A for opens or shorts, then test the communication circuit as specified by the manufacturer.
5. If power, ground, and wiring are sound, use a capable scan tool to verify that sensor A is unresponsive before replacing the sensor/module.

Common fixes

• Repair sensor A's loose, corroded, or heat-damaged connector and terminals.
• Repair an open, short, or damaged splice in the sensor A bus wiring.
• Restore the sensor module's power or ground.
• Replace exhaust pressure sensor A if it remains offline after circuit checks pass.

Can I keep driving?

If the vehicle runs normally, limit driving to a short, gentle trip and avoid towing or heavy loads, especially if exhaust or DPF warnings appear. Missing pre-DPF or pre-turbine pressure data can disrupt exhaust diagnostics and emissions control; stop if power falls sharply or the engine overheats.