U0631

Code

Generic U — Network/User

Lost Communication With Engine Coolant Temperature Sensor 5

Brand: Generic

Type: U — Network/User

Views: UK: 20 EN: 48 RU: 31

AI status

Completed

ready

Completed 100%

Page language: EN

Causes

Open or shorted wiring between ECT sensor 5 and the reporting module
Corroded, loose or damaged connector at the sensor or module
Failed ECT sensor 5 (internal open/short or intermittent)
Faulty sensor power or ground circuit (blown fuse, bad ground)
Network bus fault (CAN, LIN or dedicated serial link) preventing data transmission
Faulty reporting module (ECM/BCM/cluster) or sensor module

Symptoms

MIL (Check Engine) or network warning lamp illuminated
No temperature reading or incorrect temperature on dash/gauge cluster
Cooling fan or HVAC behavior abnormal (fan may not run or may run continuously)
Engine may run in default/limp settings related to temperature-dependent controls
Freeze frame may show missing or invalid ECT data or communication timeouts

What to check

Scan vehicle with a capable scan tool: read U0631 and any related codes, view freeze frame and live data for ECT sensor 5
Inspect sensor 5 connector and wiring for corrosion, looseness, pin damage, and chafing
Check related fuses and main power/ground circuits for continuity and proper voltage
Check for other network-related U-codes that suggest CAN/LIN bus issues
Backprobe the sensor connector and measure reference voltage, signal voltage and ground at key operating temperatures
Measure sensor resistance (if service manual provides resistance vs temperature table)

Signal parameters

Sensor element: NTC thermistor resistance varies with temperature — compare to OEM resistance vs. temperature spec (consult service manual)
Typical sensor signal voltage: 0–5 V (usually changes with temperature; verify expected idle and warm voltages in manual)
Sensor supply/reference: typically ignition-switched 5 V reference or battery via pull-up — verify present at connector
CAN bus idle voltages: each CAN line near half battery (~2.5 V) and proper differential signaling (use scope)
LIN bus idle: typically battery voltage when idle; LIN low pulses indicate communication (verify with oscilloscope)

Diagnostic algorithm

Connect scan tool; record DTCs, freeze frame and live data. Note whether sensor 5 data is missing, static, or showing communication timeouts.
Inspect the physical condition of the sensor connector, wiring harness and routing. Repair any visible damage before further testing.
Verify power and ground: with ignition on, check sensor reference voltage and ground at the connector. Repair broken supply/ground as needed.
Measure sensor resistance at ambient temperature and compare to service specification. If resistance is out of range, replace the sensor.
Backprobe the sensor signal while warming engine and watch live data for expected voltage/resistance change. If signal does not change, suspect sensor or wiring.
Perform continuity and short-to-power/ground tests on the sensor signal and reference wires. Repair any opens or shorts, then retest.
If the sensor and wiring appear correct, check the network (CAN/LIN) status: look for related bus errors, check termination and measure idle voltages. Use oscilloscope for intermittent/communication waveform faults.
If wiring and network are good, test or swap the reporting module per service procedures (or reflash/update software if applicable).
Clear codes and test drive to confirm repair; monitor for return of U0631 or related codes.

Likely causes

Broken or chafed harness to sensor 5
Corroded connector at sensor or module pins
Failed ECT sensor element
Faulty ground or sensor supply circuit

Fault status

⚠️ Status

Lost communication with Engine Coolant Temperature Sensor 5 — the control module is not receiving valid data from ECT sensor 5 (intermittent or complete loss).

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5–3.0 hours