What it means
The vehicle bus reports loss of communication with smart engine coolant bypass valve D. Bypass switching for thermal management cannot be commanded.
At a glance
- Usually lights the warning light of the system that lost communication
How it works
Smart engine coolant bypass valve D switches coolant flow for thermal management and sends status messages over the vehicle bus. U0618 sets when the control module loses communication with valve D, so it cannot reliably command or confirm that valve's bypass switching.
Common causes
Most likely first.
- Bus wiring open or shorted
- Module without power or ground
- Module failed
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Capture freeze-frame data and scan all modules for related communication faults to check whether valve D is the only device offline.
- Check live coolant-temperature data and valve D status for missing feedback or an unavailable response.
- Inspect valve D's connector and bus wiring for loose connections, corrosion, coolant contamination, chafing, or heat damage.
- Check valve D module power and ground according to the manufacturer's wiring information.
- Test the valve D communication wires for opens or shorts using the manufacturer's procedure and repair any defect.
- If power, ground, and wiring are sound, verify valve D's bus communication before replacing its module.
Common fixes
- Reconnect and secure the engine coolant bypass valve D connector.
- Clean corrosion or repair damaged terminals at valve D's connector.
- Repair an open, short, or damaged section of valve D's communication wiring.
- Restore valve D module power or ground.
- Replace the valve D module if it fails communication testing with a verified circuit.
Can I keep driving?
Drive only as needed while watching coolant temperature, since valve D's bypass switching may not be available. Stop safely if the engine temperature rises or an overheating warning appears.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.