U066C: Lost Communication With Exhaust Heat Exchanger Bypass Valve Actuator B

What it means

The vehicle bus reports loss of communication with smart exhaust heat exchanger bypass valve actuator B. Exhaust-heat recovery cannot be commanded; cabin/coolant warm-up may slow down.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

The exhaust heat exchanger bypass valve actuator B routes exhaust heat through or around the heat exchanger. The control module sets U066C when it loses the actuator's network messages, preventing it from reliably commanding that valve and managing exhaust-heat recovery.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. 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

1. Scan all modules for communication faults and review freeze-frame data to check whether other devices lost contact at the same time.
2. Check whether actuator B appears online in the scan tool and whether the module reports a commanded or actual valve position.
3. Inspect the actuator B connector and harness near the exhaust heat exchanger for looseness, corrosion, heat damage, or chafing.
4. Using the manufacturer's wiring information, verify actuator power and ground and check the network wires for opens, shorts, or poor connections.
5. If its wiring and supplies are intact, test communication at actuator B and assess whether the actuator module has failed.

Common fixes

• Reconnect or clean the exhaust heat exchanger actuator B connector and repair damaged wiring.
• Restore the actuator's power or ground connection if it is missing or poor.
• Repair an open or short in the network wiring to actuator B.
• Replace actuator B if power, ground, and network wiring are sound but the actuator remains offline.

Can I keep driving?

You can usually drive gently if the engine temperature stays normal, but expect exhaust-heat recovery and cabin or coolant warm-up to be reduced. Watch the temperature gauge and stop if the engine overheats or another warning indicates a serious fault.