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U066C Lost Communication With Exhaust Heat Exchanger Bypass Valve Actuator B

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modeNo

Exhaust heat recovery sends hot gas through a coolant heat exchanger after a cold start and later steers it past; the steering is done by bypass flaps. Actuator B is the second such flap, present only where the layout has two paths or stages. U066C is logged once that actuator, linked over a serial bus such as LIN, stops answering the controller. No maker has published a definition for this code. The nearest published check covers actuator A: GM's hybrid tables from 16OBDG02 on set U063E after 80 unanswered LIN requests at 40 ms (about 3.2 s), Type B with the MIL after two trips; no table lists a B actuator. CARB requires lost communication of digital parts to be detected separately. Hardware faults of flap B are P23A2 to P23A8. Expect slower warm-up in cold weather.

Likely causes

Actuator B electronics damaged by exhaust heat

LIN, supply or ground wire under the vehicle damaged

Water or salt corrosion in the actuator connector

Low supply voltage during the check

Symptoms

Check engine light, on some systems only after two drive cycles
Engine and cabin heater warm up more slowly
Hybrid engine runs longer to reach temperature

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€60–900

Time0.5–3 h

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

Parts involved

exhaust heat exchanger bypass actuator bvehicle bus wiring

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