U0691 is set when the powertrain control module (PCM) stops receiving periodic data messages from the exhaust pressure sensor. The code indicates a loss of communication rather than a direct sensor malfunction, meaning the problem can be in the sensor itself, its wiring/connectors, its power supply, the vehicle network, or (rarely) the PCM.
The sensor’s internal electronics can stop transmitting, causing the PCM to lose communication. This is the most common source of U0691.
Open circuits, broken wires, or corroded pins in the sensor harness interrupt the data link. Physical damage, moisture ingress, or heat‑related wear are typical culprits.
A blown fuse, faulty relay, or poor ground can prevent the sensor from powering up, resulting in no communication.
Problems on the controller area network (shorts, excessive resistance, or other bus issues) can block messages from the sensor even if the sensor itself is functional.
A hardware failure or software bug in the control module can cause it to miss valid messages. This is the least likely cause and should be investigated only after the above items are ruled out.
The vehicle may enter a reduced‑power or limp‑home mode, so continued operation is possible but not ideal. It’s best to diagnose and repair the issue promptly to avoid potential drivability or emissions problems.
No. Clearing the code only erases the stored fault; if the underlying wiring, power, or sensor issue isn’t fixed, the code will reappear after a drive cycle.
No. In the vast majority of cases, the fault lies in the sensor, its wiring, connectors, or power supply. PCM replacement is rarely required and should only be considered after all other possibilities are thoroughly investigated.