OBD-II trouble code
U0166: Lost Communication With Auxiliary Heater Control Module
A module on the network can no longer hear from the auxiliary heater control module — the controller for a supplemental cabin or coolant heater. Cabin warm-up may be slow. Usually a power, ground, fuse, wiring, or connector fault.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $80 – $900
- DIY difficulty
- Intermediate DIY
What does U0166 mean?
U0166 is set when another module on the vehicle's communication network stops receiving messages from the auxiliary heater control module. An auxiliary heater is a supplemental heat source fitted to vehicles where the engine alone warms the cabin slowly — common on diesels, hybrids, and electric vehicles, and on many cold-climate gasoline models. It may be an electric (PTC) heater element, a fuel-fired coolant heater, or a coolant-circulation heater, and a dedicated control module manages it, deciding when to add heat based on cabin demand, coolant temperature, and engine state. That module trades data with the climate system and other controllers over the bus; when the network loses contact with it, the monitoring module logs U0166.
The causes follow the standard communication-code pattern. The auxiliary heater module may have lost power or ground through a blown fuse, a corroded ground, or a wiring fault. Its connector may be corroded, loose, or have a backed-out terminal, and because the module and heater often sit low in the engine bay or under the vehicle, they are exposed to water, salt, and road debris. The CAN or bus wiring may be chafed, broken, or shorted, the module itself can fail internally, and another module on the bus can disrupt communication for everyone.
The symptoms are usually limited to heating: the cabin may warm up slowly, the supplemental heater may not engage, and on some vehicles a warning about the auxiliary or parking heater appears. The vehicle starts and drives normally because the auxiliary heater has nothing to do with propulsion, which is why U0166 is a low-severity, driveable fault. The practical downside is reduced heating and slower defrost in cold weather — worth fixing for comfort and visibility, though not a safety-critical powertrain fault.
Common causes
- Blown fuse feeding the auxiliary heater control module
- Corroded or loose ground at the auxiliary heater module
- Corroded, loose, or backed-out terminal at the module connector
- Water, salt, or road-spray intrusion at an underbody- or low-mounted module
- Chafed, broken, or shorted CAN/bus wiring to the auxiliary heater module
- Failed auxiliary heater control module
- Harness disturbed during engine-bay or underbody service
- Another module on the bus disrupting network communication
Symptoms
- Cabin warms up more slowly than normal in cold weather
- Supplemental / auxiliary heater does not engage
- Warning message about the auxiliary or parking heater on some vehicles
- Reduced defrost performance when the engine is cold
- Scan tool cannot communicate with the auxiliary heater module
- Vehicle starts and drives normally
Diagnostic steps
- 1.Scan all modules and confirm the auxiliary heater module is the unreachable one while others respond — if several modules are missing, suspect the bus itself.
- 2.Check the fuse feeding the auxiliary heater module and verify its power and ground (these heaters can draw heavy current, so check their dedicated supply).
- 3.Locate the module and heater for the specific vehicle and inspect for water, salt, and corrosion, especially on low or underbody mounts.
- 4.Inspect the module connector for corrosion, looseness, and backed-out terminals.
- 5.Inspect the CAN/bus wiring to the module for chafing, breaks, and shorts.
- 6.If power, ground, and wiring are good but the module still won't communicate, replace and configure it per the manufacturer procedure.
Repair cost
$80 – $900
A blown fuse, corroded ground, or connector repair is the cheapest fix at $80-$250 plus $120-$250 diagnostic time. Wiring repair plus cleaning corrosion runs $150-$450. Replacing the auxiliary heater control module runs $250-$800+ including any programming; fuel-fired and high-output coolant heaters and their modules can cost more, and underbody access can add labor.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.