U0618 code meaning
Lost Communication With Engine Coolant Bypass Valve D
Network system Generic code U06xx codes Check engine light
What U0618 means
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.
U0618 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0618
- Check engine light after two trips
- Unusual coolant temperature or warm-up behaviour
Common causes of U0618
- Most likelyLIN wire, supply or ground to valve D interrupted or chafed
- PossiblePoor contact at the valve connector
- PossibleValve D actuator electronics failed
- Less likelyFault on the shared LIN bus or in the LIN master (several LIN codes)
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Coolant bypass valve dVehicle bus wiring
What each part of U0618 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 18 | The exact fault in that area. |
What to do next
- Note U0618 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0618
What does U0618 mean?
U0618 means "Lost Communication With Engine Coolant Bypass Valve D". Bypass valve D is a fourth smart coolant valve and appears only on engines with several separately controlled coolant paths. It is a generic code, so it means the same on every make.
Is it safe to drive with U0618?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will U0618 fail an emissions test?
Yes, it can. U0618 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U0618 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will U0618 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.