U0634

Code

Generic U — Network/User

Lost Communication With Hybrid/EV Battery Pack Coolant Temperature Sensor A

Brand: Generic

Type: U — Network/User

Views: UK: 33 EN: 52 RU: 36

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Page language: EN

Causes

Open, short or high-resistance wiring between sensor and BPCM
Corroded, loose or damaged connector at sensor or module
Failed battery pack coolant temperature sensor (NTC/thermistor or sender)
BPCM or other network module fault (software or hardware)
CAN/LIN bus communication fault, interference, or missing message
Blown fuse, lost reference or ground to sensor circuit

Symptoms

Hybrid/EV system warning light or battery system warning lamp illuminated
Reduced battery cooling performance or fault mode for thermal management
Battery charge or HV system derate, reduced power, or limp-home behavior
No coolant temperature value shown in scan tool data for battery pack sensor A
Possible inability to start EV mode, charge, or regulate battery temperature

What to check

Read and record U0634 and any other related U or P codes and freeze frame data with a capable scan tool
Verify ability to communicate with the Battery Pack Control Module (BPCM) and read live data
Check for the presence of the battery coolant temp A data parameter in the module live data
Visually inspect sensor connector, harness, and battery pack entry points for damage, corrosion or moisture
Inspect fuses and accessory power/ground circuits for the BPCM and sensor supply
Backprobe sensor connector to measure reference voltage, signal voltage and ground continuity

Signal parameters

Sensor type: typically NTC thermistor (resistive) or resistance-based sender — values vary by manufacturer
Typical reference: 5 V reference or module-supplied pull-up (verify with service data)
Typical signal range: ~0.1–4.9 V (depends on sensor type) if sensor outputs analog voltage
Resistance example: many thermistors ≈10 kΩ at 25 °C (manufacturer-specific — confirm with data)
Network: sensor data may be sent on CAN/LIN via BPCM message ID(s); expected update rate often 1–10 Hz
Expected CAN idle: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (with bus bias; verify exact values per vehicle)

Diagnostic algorithm

Connect a factory-level or advanced scan tool. Read and record DTCs, module presence and freeze frame data.
Clear code, perform ignition cycles, and see if code returns. Note whether code is permanent, intermittent, or continuous.
Verify whether the BPCM is communicating on the network and whether the battery coolant temp A PID is present or shows open/invalid.
Visually inspect the sensor, harness, and connectors at the battery pack for corrosion, damage, water intrusion or pin push-out.
With ignition on (follow safety procedures for HV systems), backprobe the sensor connector. Confirm reference voltage, signal voltage and ground continuity to the BPCM. Compare to expected values in service data.
If sensor is a resistance type, disconnect sensor and measure resistance vs temperature (or compare to known ambient). Replace if out of spec.
Check for shorts to power/ground on the signal and reference wires and check continuity between sensor and module pins. Repair any wiring faults.
Inspect and test CAN/LIN bus if the sensor communicates indirectly: check bus voltages, termination resistors, and look for other modules with communication faults.
If wiring and sensor test good, consult module diagnostics: attempt reprogramming/reset of BPCM or related modules per manufacturer procedures and update software if available.
Replace the sensor only after confirming wiring and module communications are good. After repairs, clear codes, perform a forced test or relearn if required, then road/thermal test to verify fix.
Safety note: When working on HV battery pack components, follow manufacturer HV safety procedures and isolate high-voltage systems before accessing connectors inside the pack.

Likely causes

Damaged connector or poor pin contact at the coolant temp sensor
Open or chafed sensor signal or reference wire between sensor and BPCM
Failed coolant temperature sensor element (out of spec resistance/voltage)
Faulty ground or 5 V reference to the sensor
Battery pack control module (BPCM) not seeing or not decoding the sensor message
Network (CAN) wiring short or bus termination problem preventing message traffic