U0685: Lost Communication With Catalyst Temperature Bank 1 Sensor 2

What it means

The vehicle bus reports loss of communication with smart catalyst temperature sensor on bank 1 sensor 2. Catalyst protection and light-off monitoring switch to model-based estimates.

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

Catalyst temperature sensor 2 on bank 1 supplies temperature information over the vehicle network for catalyst protection and warm-up monitoring. This code sets when the network no longer receives messages from bank 1 sensor 2, so the controller may substitute an estimated temperature.

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. Record freeze-frame data and scan all modules for communication faults to see whether bank 1 sensor 2 is the only device offline.
2. Check live data for bank 1 sensor 2 temperature and communication status, comparing it with bank 1 sensor 1 and other available catalyst readings.
3. Inspect the bank 1 sensor 2 connector and exhaust-area bus wiring for heat damage, abrasion, corrosion, or a loose connection.
4. Check sensor or module power and ground according to the manufacturer's procedure.
5. Test the bank 1 sensor 2 bus branch for opens and shorts, then isolate the sensor or module if the wiring and supplies pass.

Common fixes

• Secure or clean the bank 1 sensor 2 connector.
• Repair damaged bus wiring or terminals near bank 1 sensor 2.
• Restore a missing sensor or module power or ground connection.
• Replace bank 1 sensor 2 or its module if circuit tests confirm an internal fault.

Can I keep driving?

A gentle, short drive may be possible if the vehicle behaves normally, but catalyst monitoring and protection may use estimated temperature. Avoid heavy loads until the fault is diagnosed, and stop if the vehicle develops drivability problems.