U0685

Code

Generic U — Network/User

Lost Communication With Catalyst Temperature Bank 1 Sensor 2

Brand: Generic

Type: U — Network/User

Views: UK: 26 EN: 32 RU: 18

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or short in sensor wiring harness (power, ground, signal)
Corroded, loose or damaged sensor connector
Failed catalyst temperature sensor (thermistor/thermocouple/IC)
Poor ground or reference voltage to the sensor
Intermittent wiring damage (chafing, heat damage)
CAN/LIN/serial bus communication fault (if sensor uses bus communications)

Symptoms

Check Engine Light (MIL) or malfunction indicator illuminated
Stored U0685 and possibly related communication codes
Possible reduced engine performance or limp-home behavior depending on strategy
Emissions readiness test failure or failed catalytic efficiency monitors
Intermittent faults that clear when connectors are moved or engine cooled

What to check

Read all stored and pending DTCs and note freeze frame/live data
Check for related communication/bus codes (U0100, U0073, etc.)
Visually inspect sensor connector and wiring at Bank 1 Sensor 2 for damage, corrosion, heat or soot
Check for proper reference voltage and ground at the sensor connector (with harness connected and key ON where applicable)
Backprobe the signal circuit and confirm presence and behaviour of expected signal (voltage, resistance, or CAN/LIN messages) with a multimeter or scope
Perform wiggle test on harness and connectors while monitoring data for intermittent faults

Signal parameters

Sensor type: typically a thermocouple, thermistor or temperature sensor IC (analog voltage or resistance) or a digital/bus message depending on vehicle
Expected analog signal behavior: temperature-dependent voltage or resistance (varies by manufacturer)
If thermocouple: low millivolt output that increases with temperature; if thermistor: resistance decreases/increases with temperature
If bus-type sensor: regular CAN/LIN messages or module messages containing temperature data
No valid signal or repeated invalid values/timeout triggers the U0685 code

Diagnostic algorithm

Retrieve all codes and freeze frame data. Note any additional communication or catalyst-related codes.
Inspect the physical condition of the Bank 1 Sensor 2 connector and wiring for corrosion, heat damage, melted insulation, or poor seating. Repair connector issues before further testing.
Identify the sensor circuit pins (signal, ground, reference/power). With ignition ON (engine off), measure reference voltage and ground integrity at the connector. Compare to expected ranges from service data.
Backprobe the signal circuit while monitoring with a multimeter or preferably an oscilloscope to confirm the presence and behavior of the temperature signal or bus messages. For bus-type sensors, use a scan tool to observe message presence on the network.
Perform continuity and resistance checks on wiring between sensor and ECM to identify opens or high resistance. Check for shorts to battery voltage and ground.
Wiggle the harness and connector under load (engine running or while monitoring live data) to reveal intermittent faults.
If wiring and connectors test good, replace the catalyst temperature sensor with a known good unit and re-check operation.
If sensor replacement does not restore communication, inspect/diagnose the vehicle network (CAN/LIN) for faults and check the ECM/module for proper operation or software updates.
After repair, clear codes and perform a drive cycle or operation that exercises the catalyst temperature sensor to verify the fault does not return.

Likely causes

Connector corrosion or pin damage at the sensor
Open/short in the sensor signal or ground circuit between sensor and ECM
Failed sensor due to heat exposure or contamination
Faulty module/ECM or lost bus communication

Fault status

⚠️ Status

Lost Communication With Catalyst Temperature Bank 1 Sensor 2 — No valid temperature data or messages received from the downstream catalyst temperature sensor on Bank 1. The control module flagged a missing or invalid signal/communication and stored U0685.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours