U0685 indicates the engine control module (ECM/PCM) has lost communication with the Bank 1 catalytic converter temperature sensor 2 (the downstream sensor located after the catalyst). This is a network communication fault, typically caused by an electrical issue rather than engine performance problems.
The Bank 1 Sensor 2 temperature sensor itself has failed electrically, preventing communication with the ECM.
Damaged, chafed, or broken wires in the sensor circuit, often from heat exposure or road debris.
Oxidation, corrosion, or poor connection at the sensor connector or ECM harness connector.
A short to ground, battery, or open circuit in the sensor signal, reference ground, or 5V reference wire.
The sensor is not receiving proper reference voltage (typically 5V) from the ECM.
Rarely, the engine control module itself may have an internal fault preventing communication with multiple sensors; this should be a diagnosis of last resort.
Yes, the vehicle will typically drive normally, but the Check Engine Light will remain on and the catalyst efficiency monitor cannot run, which will cause an emissions test failure.
Not directly. U0685 is a communication fault with the temperature sensor, not an indication of catalytic converter failure. However, a failed sensor will prevent the ECM from monitoring converter efficiency.
No. While a failed ECM can cause U-codes, it is the least likely cause. The overwhelming majority of U0685 codes are caused by the failed sensor, wiring issues, or poor connections—not the control module. Replacing the PCM is almost never the correct repair for this code.
Bank 1 refers to the cylinder bank containing cylinder #1 (the side with cylinder #1 in V-type engines). Sensor 2 is the downstream temperature sensor located after the catalytic converter, used to monitor converter efficiency.