U0635 indicates that the Powertrain Control Module (PCM) has lost communication with the hybrid‑EV battery pack coolant temperature sensor. The sensor provides the battery’s coolant temperature to manage thermal control and charging strategies.
The sensor itself can fail open, short, or provide out‑of‑range values, breaking the communication link.
Vibration, moisture, or age can degrade the connector pins or housing, causing intermittent loss of signal.
Chafed, cracked, or shorted wires in the harness can interrupt the sensor’s data line.
A malfunction within the battery’s own control electronics can prevent it from responding to the PCM’s request.
Only after ruling out sensor, wiring, and battery‑module issues should a PCM internal failure be considered.
In most cases no. The PCM is rarely the cause; the sensor, its connector, or wiring are far more common culprits. Replace the PCM only after all other possibilities have been eliminated.
If the loss of communication was caused by an intermittent connector or wiring issue, the code may clear after a successful communication cycle, but the underlying fault will likely return.
No. The coolant temperature sensor monitors the temperature of the battery‑coolant loop, while the battery temperature sensor measures the cells directly. U0635 specifically refers to the coolant sensor.
A fault in the battery pack’s control module can prevent it from responding to the PCM, which can trigger U0635, but this is less common than a sensor or wiring problem.