Code U0631 indicates that the engine control module (PCM) has lost its communication link with Engine Coolant Temperature Sensor 5. This means the PCM can no longer receive temperature data from this specific sensor, which can disrupt engine cooling fan operation and fuel mixture control.
The internal electronics of the sensor itself have failed, preventing it from transmitting data over the communication network.
An open circuit, short to ground, or corroded connector interrupts the communication, power, or ground wires between the sensor and the PCM.
Problems on the specific data line (such as a SENT or LIN bus) used by the sensor to communicate with the PCM can cause a total loss of signal.
A lack of proper voltage or ground supply to the sensor will cause the sensor to go offline, resulting in a loss of communication.
A rare scenario where the internal communication controller or driver inside the PCM has malfunctioned, though this is almost never the root cause.
Modern vehicles often use multiple temperature sensors for different systems, cooling circuits, or engine banks. Sensor 5 indicates this is a secondary or auxiliary sensor rather than the primary one.
It is not recommended. Without accurate temperature data, the engine may run poorly, overheat, or the cooling fans may not operate correctly, potentially leading to severe engine damage.
It is highly unlikely. While some mechanics or online advice may suggest replacing the PCM, the cause is almost always a failed sensor or a wiring issue. The PCM should only be considered as a last resort after all wiring and the sensor itself are confirmed to be functioning.
Disconnecting the battery might clear the code temporarily, but it will return immediately if the communication loss is still present. The underlying fault must be repaired.