What Does U0166 Mean?
Communication has been lost between the main control modules and the auxiliary heater control module. This module typically controls supplemental heating systems such as diesel-fired heaters or electric heating elements.
Common Causes
40%
Failed auxiliary heater control module
30%
Damaged wiring or connectors to auxiliary heater module
15%
Loss of power or ground to auxiliary heater control module
10%
CAN bus communication circuit fault
5%
Software issue or module not properly configured
Diagnostic Steps
1
Step 1: Verify power and ground at auxiliary heater control module using a multimeter
2
Step 2: Inspect all connectors and wiring to the auxiliary heater module for physical damage or corrosion
3
Step 3: Check CAN bus communication signals at the auxiliary heater module connector (CAN High and CAN Low)
4
Step 4: Test auxiliary heater control module by substitution if available, or monitor module response with scan tool
5
Step 5: Review vehicle-specific service information for auxiliary heater system diagnosis and module programming requirements
Estimated Repair Cost
$200 - $900
Parts + labor, varies by vehicle and location
Communication has been lost between the main control modules and the auxiliary heater control module. This module typically controls supplemental heating systems such as diesel-fired heaters or electric heating elements.
The most common cause of U0166 (Lost Communication With Auxiliary Heater Control Module) is: Failed auxiliary heater control module
Typical repair costs for U0166 range from $200 to $900, depending on the vehicle, location, and whether you do it yourself or go to a shop.
No safety concern. Supplemental heating will not function but primary heating system typically remains operational.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to U0166 to identify the root cause.
OBDHut Mobile App
Scan codes directly from your car with the OBDHut app.
Coming Soon
Quick Info
Category
Network
System
Climate Control / Auxiliary Heating
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
A quality scan tool helps you read codes, view live data, and clear faults.
