U0633 indicates that the vehicle's control module has lost communication with the second engine cooling fan (Fan 2). This typically means the PCM/ECM is not receiving expected signals from the fan circuit, which can prevent the fan from operating when needed.
The fan motor itself may have an open winding, short, or mechanical seizure, preventing it from responding to the PCM's command.
Corrosion, chafing, or loose pins in the fan‑2 circuit can interrupt the signal or power supply, causing a communication loss.
If the relay that powers fan 2 sticks open or the fuse blows, the PCM will not detect the fan's feedback and will set U0633.
A fault on the vehicle's CAN network (e.g., terminated incorrectly, bus short, or noisy signal) can prevent the fan module's messages from reaching the PCM.
Only if the internal driver that commands fan 2 is defective will the PCM be at fault; this is uncommon and should be considered after wiring and fan checks.
PCM replacement is rarely required for this code. Most U0633 cases are due to wiring, connector, fuse, relay, or fan‑2 motor faults. Only consider the PCM after those components have been tested and found good.
Driving may be possible for short distances, but the lost fan 2 can lead to higher engine temperatures, especially in traffic or hot weather, increasing the risk of overheating. Monitor temperature gauge and avoid prolonged high‑load operation.
You can apply 12 V directly to the fan‑2 power feed (using a fused jumper) to see if the motor spins. If it runs, the issue is likely in the control circuit; if it does not, the motor or its supply is faulty.
Start with the simplest items: inspect the fan‑2 fuse and relay, then check the wiring harness and connectors for damage or corrosion before moving to motor or bus testing.