U0684Lost Communication With Catalyst Temperature Bank 1 Sensor 1
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- A catalyst temperature sensor tells the controller when the converter is hot enough to work and to be tested. Sensor 1 on bank 1 is the first such probe on the bank that holds cylinder 1. U0684 applies when the probe sends its reading as a digital message and that message has stopped. No manufacturer documents this code publicly, and many makers fit no sensor here: Ford's 2024 gasoline summary works with an inferred mid-bed temperature and only runs its catalyst test between 510 and 815 °C of that model value. CARB accordingly lets the catalyst temperature in the data stream be measured or estimated, but wants a separate code for every silent digital input. Analog circuit faults at this point are P0425 to P0428; U0685 covers sensor 2.
- What you may notice
- Check engine light
- Catalyst readiness monitor does not complete
- Likely causes
- LikelySensor lead near the converter burnt or chafed
- PossibleEvaluation electronics of the temperature sensor failed
- PossibleCorroded connector under the vehicle
- RareEngine controller input or software fault
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €60 – €450
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.