U0684: Lost Communication With Catalyst Temperature Bank 1 Sensor 1

What it means

The vehicle bus reports loss of communication with smart catalyst temperature sensor on bank 1 sensor 1. 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 1 on bank 1 reports temperature over the vehicle network for catalyst protection and warm-up monitoring. This code sets when messages from that specific sensor stop arriving, so the controller may estimate the temperature instead of using its reading.

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, noting whether the fault is limited to bank 1 sensor 1 or affects other network devices.
2. Check live data for bank 1 sensor 1 temperature and communication status, comparing it with bank 1 sensor 2 or bank 2 readings if available.
3. Inspect the bank 1 sensor 1 connector and bus wiring near the exhaust for heat damage, chafing, corrosion, or loose terminals.
4. Verify the sensor or module's power and ground using the manufacturer's procedure.
5. Test the sensor's bus branch for opens or shorts, then assess the sensor or module if wiring and supplies are intact.

Common fixes

• Reconnect or clean the bank 1 sensor 1 connector.
• Repair heat-damaged or chafed bus wiring and terminals for bank 1 sensor 1.
• Restore the sensor or module's power or ground.
• Replace bank 1 sensor 1 or its module if testing confirms an internal failure.

Can I keep driving?

If the vehicle runs normally, a short, gentle trip may be possible, but catalyst temperature protection and monitoring may rely on estimates. Avoid hard acceleration or towing until diagnosed, and stop if drivability or warning conditions worsen.