U0686 indicates that the powertrain control module (PCM) has lost communication with the catalyst temperature sensor on bank 2, sensor 1. This sensor monitors exhaust‑catalyst temperature for emissions control, and a loss of its signal can affect catalyst efficiency monitoring and trigger the malfunction indicator lamp.
Physical damage, moisture intrusion, or corrosion on the sensor harness or connector can interrupt the communication line.
The sensor itself may have an internal open or short that prevents it from transmitting data.
A break or short to ground/power in the signal wire will stop communication between the sensor and PCM.
Inadequate grounding or a voltage drop on the sensor supply can cause loss of communication.
A fault on the CAN or dedicated sensor bus within the PCM can prevent data receipt from the sensor.
Rarely, the PCM’s input circuitry for this sensor may be defective, but this is a low‑probability cause.
No. The PCM is rarely the cause; most often the issue is wiring, connector, or sensor failure. PCM replacement should only be considered after all other causes are ruled out.
Typically the effect is limited to emissions monitoring; drivability may be unchanged, though fuel economy can suffer slightly.
The vehicle can usually be driven, but the emissions system may not operate correctly and the vehicle may fail an emissions test.
A scan tool capable of reading live data, a digital multimeter for circuit testing, and the vehicle’s wiring diagram are sufficient.