U0684

Code

Generic U — Network/User

Lost Communication With Catalyst Temperature Bank 1 Sensor 1

Brand: Generic

Type: U — Network/User

Views: UK: 17 EN: 36 RU: 23

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or short in sensor signal, power, or ground circuit
Corroded, loose, or damaged connector at the sensor
Failed catalyst temperature sensor
Faulty or intermittent module (ECM/TCM/aftertreatment controller) receiving the sensor
Network communication fault (CAN/LIN) between modules
Software or calibration error in control module

Symptoms

Check Engine Light / MIL illuminated
Possible reduced emissions system performance or limp mode on some vehicles
Stored U0684 and possibly related network U-codes
Failed readiness for catalyst monitor; vehicle may not pass emissions test

What to check

Scan for U0684 and any related codes (other U-codes, P0xxx/P2xxx/P04xx). Record freeze frame and live data.
Inspect the catalyst temperature sensor harness and connector for heat damage, corrosion, or breaks.
Back-probe sensor connector and check for proper reference power and ground (if applicable).
Check for CAN/LIN network errors and other module communication faults with a diagnostic tool.
Wiggle test wiring with engine running (or key on) while watching live data / DTCs for intermittent faults.

Signal parameters

Sensor type: often thermistor/thermocouple or exhaust temp sender—signal can be analog voltage, resistance change, or digital message
Typical analog ranges: 0–5 V signal (varies by manufacturer); no communication or out-of-range voltage will set the code
Resistance/voltage vs temperature: resistance decreases/increases with temp depending on sensor type — compare to manufacturer specification
Networked sensors may use periodic CAN/LIN messages instead of a simple analog signal—message frequency typically 1 Hz or faster
Expected live-data: stable temperature reading that changes with engine load/exhaust flow; invalid or no value indicates fault

Diagnostic algorithm

Retrieve freeze-frame and full code list with a scan tool. Note any additional U- or P-codes that could indicate a broader network or sensor issue.
Visually inspect the Bank 1 Sensor 1 catalyst temperature sensor and harness from the sensor to the next connector and to the module. Look for heat damage, chafing, burns, pinched wires, or corrosion.
With ignition ON (engine off) or engine running as required, back-probe the sensor connector and check signal, reference power (if present), and ground with a multimeter. Compare to expected ranges or behavior (signal should respond to temperature changes).
Perform a wiggle test along the harness and connector while watching live data or DTC status to try to reproduce the communication loss.
If sensor uses CAN/LIN messaging, check module communication status and bus integrity (bus voltage, termination, other module responses). Repair any bus faults first.
If wiring and connectors are good, remove the sensor and measure resistance (for thermistor type) or continuity per manufacturer spec. Replace the sensor if out of spec or damaged.
If sensor and wiring test good, test or replace the receiving control module only after verifying communication lines and checking for software updates or Technical Service Bulletins.
Clear codes and perform a drive cycle to confirm repair. Re-scan and recheck readiness status.

Likely causes

Wiring harness damage near exhaust or heat-exposed areas
Connector corrosion from exhaust condensate or road contamination
Sensor element failed due to thermal or mechanical damage
Poor ground at engine or exhaust sensor ground point
Intermittent harness contact caused by vibration or routing near moving parts

Fault status

⚠️ Status

Lost Communication With Catalyst Temperature Bank 1 Sensor 1 — the control module is not receiving valid data from the Bank 1 catalyst temperature sensor.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours