B0408: Temperature Control (Air Mix) Circuit Malfunction
2 vehicles · 1 makes · $150-$650 repair
Most Reported On
B0408 is an OBD-II diagnostic trouble code meaning “Temperature Control (Air Mix) Circuit Malfunction.” B0408 is a body (B) code relating to the climate control system's temperature/air-mix control circuit. It generally means the HVAC module detected a fault in the circuit that controls the air-mix (blend) door, which regulates how much air is blended for the desired cabin temperature. Exact wording is somewhat manufacturer-specific, but it typically results in inability to control temperature on one or both sides. This is a comfort-system fault and not a safety or drivability hazard. This code is most commonly reported on 2007-2019 GMC Sierra 3500HD, and 2010-2017 GMC Terrain (SLE, SLT, Denali), with repair costs ranging from $150 to $650.
Common Causes
Typical Repair Cost
Based on 2 documented vehicle-specific issues. Actual cost depends on root cause and vehicle.
Diagnostic Tools
To diagnose B0408, you'll need an OBD-II scanner. Here are our recommendations at every price point.
ANCEL AD310 Classic OBD-II Scanner
by ANCEL
A simple, affordable code reader that reads and clears check engine codes. Great for quick diagnostics on any OBD-II vehicle (1996+).
- Read & clear engine codes
- View freeze frame data
- I/M readiness status
- No batteries or app needed
BlueDriver Pro Bluetooth Scanner
by BlueDriver
Bluetooth OBD-II scanner with a free companion app. Provides enhanced diagnostics, smog readiness, and repair reports sourced from a database of verified fixes.
- Enhanced diagnostics (ABS, SRS, transmission)
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- Smog check readiness
- Free app (iOS & Android)
LAUNCH CRP123X OBD-II Scanner
by LAUNCH
A professional-grade handheld scanner that reads all four major systems (engine, transmission, ABS, SRS) with live data streaming and graphing.
- Engine, transmission, ABS, SRS diagnostics
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- AutoVIN for vehicle identification
- Free lifetime updates via Wi-Fi
Autel MaxiCOM MK808S Diagnostic Tool
by Autel
Shop-level diagnostic tablet with bi-directional control, active tests, and full system coverage. Ideal for serious DIYers and small shops.
- All-system diagnostics (25+ modules)
- Bi-directional control & active tests
- Oil reset, EPB, BMS, TPMS, injector coding
- 7-inch touchscreen with Android OS
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FAQ
What does B0408 mean?
B0408 stands for "Temperature Control (Air Mix) Circuit Malfunction." B0408 is a body (B) code relating to the climate control system's temperature/air-mix control circuit. It generally means the HVAC module detected a fault in the circuit that controls the air-mix (blend) door, which regulates how much air is blended for the desired cabin temperature. Exact wording is somewhat manufacturer-specific, but it typically results in inability to control temperature on one or both sides. This is a comfort-system fault and not a safety or drivability hazard.
What are the most common causes of B0408?
The most common causes of B0408 are: Failed air-mix (blend door) actuator/servo motor, Broken or jammed blend door, Wiring or connector fault in the actuator circuit, Loss of power or ground to the actuator, Faulty HVAC control module, Calibration/positioning fault after battery disconnect. The specific cause varies by vehicle.
How much does it cost to fix B0408?
Repair costs for B0408 range from $150 to $650, depending on the vehicle and root cause.
Which vehicles are affected by B0408?
Au7o has documented B0408 across 2 vehicle models from 1 manufacturers: GMC.
Sources (4)
- ForumReddit r/MechanicAdvice search results for GMC Sierra blend door actuator clicking and failure discussion
- articleGMC Terrain HVAC Blend Door Actuator Replacement Cost - RepairPal
- Forum2010 Terrain Blend Door Actuator? - Terrain Forum
- articleSolve GM Temperature Control and Blend Door Issues - FixMyOldRide
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.