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U0618 Lost Communication With Engine Coolant Bypass Valve D

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

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.

Likely causes

LIN wire, supply or ground to valve D interrupted or chafed
Poor contact at the valve connector
Valve D actuator electronics failed
Fault on the shared LIN bus or in the LIN master (several LIN codes)

Symptoms

  • Check engine light after two trips
  • Unusual coolant temperature or warm-up behaviour

Repair estimate

DifficultyMediumDIY possible
Parts and labour€30–700
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

coolant bypass valve dvehicle bus wiring

Related codes

U0615U0616U0617U063BP0128

References: gsi.ext.gm.com, en.wikipedia.org, law.cornell.edu.