B0408: How to Diagnose & Fix — Temperature Control (Air Mix) Circuit Malfunction
8 vehicles · 4 makes · $100-$650 repair
How to diagnose and fix B0408: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
GMC Sierra 3500HD · 2007–2019
Sierra 3500HD owners report clicking/ticking behind the dash and inability to control cabin temperature due to HVAC blend door or mode door actuator failures. GM service bulletins and owner reports describe stripped actuator gears or actuator motor failures that can leave the truck stuck on hot, cold, defrost, or a single vent mode.
Common Symptoms
- Clicking/ticking from dash when starting truck or changing temp
- Stuck on hot or stuck on cold air
- Air only comes from defrost or only from vents
- Inconsistent temperature left-to-right (dual zone)
- HVAC settings change but airflow/temperature does not
How to Fix
Identify the failed actuator (temperature blend, mode, or recirculation) using HVAC actuator recalibration and scan tool data where available. Replace the specific actuator and perform HVAC recalibration procedure after installation. If doors bind, inspect HVAC case/door movement before installing a new actuator.
Sources for this issue (1)
GMC Terrain · 2010–2017
The HVAC blend/mode-door actuators fail as their internal plastic gears strip, producing a repetitive clicking or knocking sound from behind the dash and causing loss of temperature or airflow control. On dual-zone systems one side commonly blows hot while the other blows cold, or the system sticks on one temperature. A click/pop typically means a stripped gear requiring replacement.
Common Symptoms
- Repetitive clicking/knocking behind dash
- One side hot, other side cold
- Stuck on cold or hot air
- Temperature setting has no effect
- No airflow mode change
How to Fix
Replace the failed blend/mode-door actuator (OEM/ACDelco recommended to avoid repeat aftermarket failures) and recalibrate with a scan tool after installation. The temperature (blend) actuator is the most common failure; mode/recirc actuators can also fail.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- HVAC Blend/Mode Door Actuator (1st design, 5-pin connector) — 2010 through early-2013OEM25952814 (ACDelco 15-73981)Cross-refDorman 604-185, Four Seasons 73253, SKP SK604185$13–$56
GM used ONE common actuator in several positions in this HVAC case (temperature/blend, mode, and air-inlet/recirculation), so this single part number covers whichever one is clicking — the temperature/blend actuator is the most common fa…
- HVAC Blend/Mode Door Actuator (2nd design, 6-pin connector) — 2013 through 2017OEM22877649 (ACDelco 15-74254)Cross-refDorman 604-180, Four Seasons 73251, GPD 1712923$13–$40
Same one-part-fits-several-positions arrangement as the early design: temperature/blend, mode, and air-inlet/recirculation actuators in this case all use this number.
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Sources for this issue (1)
GMC Acadia · 2007–2021
The dual-zone HVAC case uses several small plastic-geared blend/mode door actuators. The gear teeth strip or the door binds, so the actuator hunts for its calibration position and produces a repetitive clicking or ticking from behind the dash or glovebox. The classic complaint is full heat on the driver's side and ice-cold air on the passenger's side (or the reverse), often with the click audible whenever the temperature setting is changed or the ignition is cycled. It is one of the most common non-powertrain complaints on the Acadia across both generations because every part of the actuator drivetrain is plastic.
Common Symptoms
- Repetitive clicking or ticking behind dash or glovebox
- Heat on one side, cold air on the other
- Temperature does not change when adjusted
- Air comes from wrong vents
- Clicking stops when HVAC is off
How to Fix
Identify the failed actuator by moving the temperature setpoint and feeling/listening at each actuator — the failed one clicks while the good one is smooth. The passenger blend actuator is reachable from behind the glovebox and is held by three T20 Torx screws; some mode actuators require dropping more of the lower dash. Replace the actuator, then start the vehicle and let the HVAC control module run its self-calibration for roughly 40 seconds without touching the controls. Inspect the blend door shaft for binding or a broken door before installing a new actuator, or the new one will strip too.
Sources for this issue (3)
Chevrolet Impala · 2006–2016
Buick Enclave · 2008–2017
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B0408 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
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 $100 to $650, depending on the vehicle and root cause.
Which vehicles are affected by B0408?
Au7o has documented B0408 across 8 vehicle models from 4 manufacturers: Buick, Cadillac, Chevrolet, GMC.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.