Network, generic

U0685 Lost Communication With Catalyst Temperature Bank 1 Sensor 2

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modeNo

Where a converter carries two temperature probes, one usually sits ahead of the brick and one behind it, so that the heat released by the reaction shows up as a rise between them. Sensor 2 on bank 1 is the second, downstream position. U0685 means this probe reports over a digital link and its messages no longer arrive. No manufacturer documents this code publicly, so thresholds and debounce times are unknown. Without the second reading, the comparison with sensor 1 is lost: the correlation code P046A cannot be judged, and any strategy that uses the temperature rise as a sign of catalyst activity loses its input. Analog faults of the same probe are P042A to P042D; sensor 1 has U0684. CARB requires a separate code for digital inputs that fall silent.

Likely causes

Lead of the downstream probe damaged by heat, stones or road salt

Sensor electronics failed

Connector corroded or not fully latched

Symptoms

Check engine light
Catalyst monitor or sensor comparison stays incomplete

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 2vehicle bus wiring

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