Network, generic

U0684 Lost Communication With Catalyst Temperature Bank 1 Sensor 1

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modeNo

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.

Likely causes

Sensor lead near the converter burnt or chafed

Evaluation electronics of the temperature sensor failed

Corroded connector under the vehicle

Engine controller input or software fault

Symptoms

Check engine light
Catalyst readiness monitor does not complete

Repair estimate

DifficultyMediumDIY possible

Parts and labour€60–450

Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

catalyst temp sensor bank 1 sensor 1vehicle bus wiring

References: law.cornell.edu, fordservicecontent.com, en.wikipedia.org.