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Home / DTC / U0639 — Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B

U0639 — Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B

Detailed page for trouble code U0639.

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U0639

Generic U — Network/User

Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B

Brand: Generic
Type: U — Network/User
Views: UK: 16 EN: 22 RU: 18
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Page language: EN

Causes

  • Open, shorted or damaged sensor wiring harness or connector
  • Corroded or loose connector pins at the sensor or controller
  • Failed coolant temperature sensor (sensor module) or internal electronics
  • Faulty hybrid/EV control module or sensor module
  • Loss of reference power or ground to the sensor
  • Bus communication fault (CAN/LIN) or missing termination

Symptoms

  • Malfunction Indicator Lamp (MIL) or hybrid system warning light
  • Hybrid system may enter restricted operation or limp mode
  • Loss of accurate coolant temperature readings for hybrid subsystem
  • Fans, heaters or battery thermal management may operate incorrectly
  • Intermittent or permanent fault codes stored; possible erratic temperature display

What to check

  • Read and record all stored and pending codes and freeze-frame data with a capable scan tool
  • Visually inspect sensor B connector, pins and harness for corrosion, damage, pin backout or water intrusion
  • Check relevant fuses and relays that supply the sensor/module power
  • Check for presence of reference voltage/power and ground at the sensor connector
  • Scan/check bus traffic (CAN or LIN) for the sensor module using a scan tool or oscilloscope
  • Perform a wiggle test of harness while monitoring the network/messages and live sensor data

Signal parameters

  • Consult OEM wiring diagram/specs for exact pin functions and voltages
  • Typical passive coolant temp sensors (NTC thermistors): resistance decreases with higher temp — example ~1–5 kΩ at ~20–25°C (varies by sensor)
  • Typical analog sensor voltage (if present) will vary with temperature — often in the range ~0.5–4.5 V (vehicle-specific)
  • Digital/smart sensor modules communicate over a vehicle bus (CAN/LIN) — expect normal bus idle voltages and regular messages; absence or corrupt messages indicates communication fault
  • CAN bus physical layer: each CAN line nominally near Vbatt/2 on recessive state — use OEM spec for exact thresholds

Diagnostic algorithm

  1. Connect a manufacturer-level scan tool or capable OBD-II tool. Read and record U0639 and any additional codes/freeze frame data.
  2. Verify whether the code is current, pending or intermittent. Attempt to reproduce the fault with functional tests (key cycles, temperature changes, vehicle movement if safe).
  3. Visually inspect sensor B connector and harness. Look for corrosion, bent pins, moisture, pinch points, repairs or splices.
  4. With ignition on (or per OEM procedure) backprobe the sensor connector: verify reference power, ground, and signal lines per OEM values. Do not short terminals. If voltages are absent, trace power/ground and check fuses/relays.
  5. If the sensor is a thermistor type, measure sensor resistance at known ambient temperature and compare to OEM chart. If values are out of expected range, suspect sensor or wiring.
  6. If the sensor is a smart module on a communication bus, verify bus integrity. Use a scan tool or scope to confirm the sensor module is transmitting messages and that CAN/LIN voltages and waveforms are within spec. Check for bus shorts, opens or missing termination.
  7. Perform continuity and short-to-power/ground checks on the harness between the sensor and hybrid control module. Repair any damaged wires or connectors.
  8. If wiring and bus are good but communication still fails, substitute a known-good sensor/module if available or test with OEM diagnostic procedures to determine module failure.
  9. After any repair or replacement, clear codes, cycle ignition and test drive or run system self-tests to confirm the issue is resolved and that U0639 does not return.
  10. If fault persists after sensor and wiring verification, follow OEM guidance for testing/replacing the hybrid control module or bus components.

Likely causes

  • Corroded/loose connector at the coolant temp sensor B or its module
  • Broken/shorted wire in the harness between sensor B and hybrid control module
  • Failed sensor module (electronic sensor rather than simple thermistor)
  • Fault in CAN/LIN bus segment serving the sensor (open/short or missing termination)

Fault status

⚠️ Status
Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor B — Hybrid control module is not receiving valid data or messages from coolant temperature sensor B or its module.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.5-3.0 hours

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U0100 U0121 U0131 U0141 P0117 P0118
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