A failed or intermittent catalyst temperature sensor (bank 1 sensor 2) that stops transmitting data on the network.; Damaged, corroded, or chafed wiring in the sensor harness or its connector, interrupting the CAN signal or power supply.; A poor ground connection at the sensor or at the module that interfaces with it, causing the signal to drop out.; A faulty control module (ECM or emissions module) that has lost the ability to receive or process the sensor message.; A CAN bus fault such as a shorted or open data line between the sensor interface and the module, often shared with other network codes.; Low battery voltage or a weak charging system that causes modules to reset and drop communication intermittently.; Aftermarket electrical accessories or improper jump-starting that introduce voltage spikes and disrupt network communication.

DTC codes / U06xx · Network communication / U0685

U0685Lost Communication With Catalyst Temperature Bank 1 Sensor 2

U0685 is a network communication fault indicating that a control module on the vehicle's controller area network (CAN) bus has stopped receiving messages from the catalyst temperature sensor located downstream of the bank 1 catalytic converter. The module that monitors this sensor, typically the engine control module (ECM) or a dedicated emissions control module, has detected a loss of the periodic data signal that the sensor or its interface circuit is supposed to broadcast.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaNetwork communication

Symptoms you may notice

Illumination of the malfunction indicator lamp (MIL) on the dashboard, often with no other noticeable symptoms.
A stored U0685 code accompanied by other communication or emissions-related codes in the same module.
Possible rough running or reduced engine performance if the ECM enters a default strategy that affects fuel or spark control.
Intermittent operation of the catalyst monitor or readiness monitors, preventing an emissions test from completing.
No visible change in driveability in many cases, because the fault is a data loss rather than a direct sensor failure.

Common causes, most likely first

A failed or intermittent catalyst temperature sensor (bank 1 sensor 2) that stops transmitting data on the network.
Damaged, corroded, or chafed wiring in the sensor harness or its connector, interrupting the CAN signal or power supply.
A poor ground connection at the sensor or at the module that interfaces with it, causing the signal to drop out.
A faulty control module (ECM or emissions module) that has lost the ability to receive or process the sensor message.
A CAN bus fault such as a shorted or open data line between the sensor interface and the module, often shared with other network codes.
Low battery voltage or a weak charging system that causes modules to reset and drop communication intermittently.
Aftermarket electrical accessories or improper jump-starting that introduce voltage spikes and disrupt network communication.

How to diagnose and fix it

Connect a scan tool and record all stored codes, freeze-frame data, and network communication status to confirm U0685 is active and not a history code.
Inspect the catalyst temperature sensor connector and harness for corrosion, loose pins, chafing, or heat damage, and repair any obvious wiring faults.
Check battery voltage and charging system output, as low voltage can cause intermittent communication loss across multiple modules.
Perform a resistance and voltage check on the sensor circuit according to the manufacturer's procedure, looking for opens, shorts, or out-of-range values.
If the sensor signal is absent but wiring and power are good, substitute a known-good sensor or perform a bidirectional test to confirm the sensor is the source of the communication loss.
Verify CAN bus integrity with a scope or network test, checking for proper differential voltage and termination resistance on the data lines.
Clear codes and road-test the vehicle through a complete drive cycle to confirm the communication fault does not return before releasing the vehicle.

Why a sensor code is really a network fault

U0685 is often misdiagnosed as a simple sensor replacement, but the code actually reports a loss of network messages, so the root cause can be a module, wiring, or power issue rather than the sensor itself.
A weak or failing battery is a common hidden cause of U-codes because modules drop offline during cranking or voltage dips, and the communication loss is logged before the battery finally fails.
Heat damage to the downstream catalyst sensor harness is frequently overlooked, since the sensor sits close to the exhaust and the insulation can crack and short the CAN lines intermittently.
This code can set alongside other U-codes for different modules when a shared CAN bus segment or ground point is faulty, so always check for multiple communication codes before condemning a single component.
Some vehicles will run normally with U0685 stored because the ECM substitutes a calculated catalyst temperature, masking the fault until an emissions test or a related driveability issue appears.

Can I keep driving with U0685?

The vehicle is generally safe to drive short distances because the fault affects emissions monitoring rather than immediate safety systems, but avoid long trips or towing until the cause is found. If the engine runs rough, the MIL flashes, or other modules begin dropping offline, stop driving and have the network fault diagnosed promptly.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.