B0408 on GMC
Temperature Control (Air Mix) Circuit Malfunction
B0408 on GMC vehicles indicates temperature control (air mix) circuit malfunction. Au7o has documented this code across 1 GMC model — most commonly on Sierra 3500HD. 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. Typical repair costs on GMC range from $180 to $650, depending on the specific model and root cause.
Common Causes of B0408
- •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
B0408 on GMC by Model
GMC Sierra 3500HD(1 issue)
- Blend door actuator failure causing temperature/airflow control problems2007-2019
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.
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View B0408 across all makes →Frequently Asked Questions
What does B0408 mean on GMC?▼
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. On GMC specifically, this code is documented across 1 model.
What causes B0408 on GMC vehicles?▼
Common causes on GMC: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix B0408 on a GMC?▼
Repair costs on GMC range from $180 to $650, depending on the specific model and root cause.
Which GMC models have B0408 documented?▼
Au7o has documented B0408 on 1 GMC model: Sierra 3500HD.