U0618 is a network communication fault indicating the powertrain control module (PCM) has lost CAN bus communication with the engine coolant bypass valve D. This is a circuit/messaging failure between modules, not a direct sensor or actuator malfunction.
The valve's internal electronics have failed, preventing it from responding on the CAN bus network.
Damaged, chafed, or corroded wiring between the PCM and the valve, or loose connector pins.
Blown fuse, failed relay, or interrupted power circuit providing voltage to the bypass valve.
Corroded, damaged, or loose ground wire/connection for the coolant bypass valve circuit.
Short or open in the CAN bus wiring harness affecting communication with this device.
Rare - software glitch or outdated calibration may cause communication errors.
Extremely rare - the PCM itself has failed and is not transmitting or receiving CAN messages correctly. This is almost never the cause.
Driving is possible but not recommended for extended distances. The engine may run in limp mode with reduced power, and cooling system regulation may be compromised, potentially leading to overheating.
Almost never. The PCM is very rarely the cause of this communication loss. In the vast majority of cases, the problem is the failed valve itself, wiring issues, or power/ground problems. Be wary of shops that immediately suggest PCM replacement.
Costs vary significantly by vehicle and location. The coolant bypass valve itself may range from $50 to $300+ depending on the vehicle. Wiring repairs are usually labor-intensive. Avoid expensive PCM replacements unless all other causes have been definitively ruled out.
You can clear the code, but if the underlying communication issue is still present, it will return. The code indicates a real hardware or wiring problem that should be diagnosed rather than masked by repeatedly clearing it.