U0618Lost Communication With Engine Coolant Bypass Valve D
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- Bypass valve D is a fourth smart coolant valve and appears only on engines with several separately controlled coolant paths. GM's 2025 table 25OBDG03B is the one public document found that uses U0618: the engine controller polls the valve over LIN and expects its response frame (BRV_Rsp). If that reply is missing for 250 ms or longer while the LIN channel is initialised, the valve is configured as present and the battery has held above 11 V for five seconds, the code fails as Type B and needs two trips to light the MIL. A single silent valve with working neighbours narrows the search to its own branch, connector or electronics; several LIN codes at once point to the shared bus or the master. Coolant flow in that branch may stay at its default position.
- What you may notice
- Check engine light after two trips
- Unusual coolant temperature or warm-up behaviour
- Likely causes
- LikelyLIN wire, supply or ground to valve D interrupted or chafed
- PossiblePoor contact at the valve connector
- PossibleValve D actuator electronics failed
- RareFault on the shared LIN bus or in the LIN master (several LIN codes)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €30 – €700
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.