B0408 on Chevrolet: How to Diagnose & Fix
Temperature Control (Air Mix) Circuit Malfunction
3 Chevrolet models · 3 documented issues · $100-$500 repair
B0408 on Chevrolet by model: symptoms, diagnosis and fix
A repetitive clicking or knocking sound from behind the dash — often constant, sometimes only when the car is running or turned off — is a classic Impala blend door actuator failure. The plastic gears inside the small actuator motor strip out, so the actuator can no longer position the blend door. On dual-zone systems there are separate left and right actuators; a stuck door means one side blows hot while the other blows cold, or the temperature won't respond to the setting.
Common Symptoms
- Repetitive clicking/knocking behind the dash
- One side blows hot, the other cold (dual-zone)
- Temperature doesn't change with setting
- Air comes from wrong vents
- Clicking continues after engine off
How to Fix
Locate the clicking actuator (temperature/blend, mode, or recirc — the Dorman 604-108 covers common Impala blend-door applications). Replace the failed actuator; pulling its fuse only silences the clicking temporarily and is not a fix. Many Impala actuators self-calibrate on install: disconnect the battery before installing the actuator, then set temperature to full hot or cold, reconnect the battery, cycle the key on, and let the motors auto-calibrate.
Sources for this issue (1)
The plastic gears inside the HVAC blend-door actuator strip over time, so the actuator motor spins without moving the temperature-mix door. Owners hear a repetitive clicking or knocking behind the dash (often on start-up or when changing temperature) and get uneven output such as cold air on one side and warm on the other, or the temperature not matching the setting. This is a common, well-documented Camaro complaint on Camaro5 and general GM HVAC references, mirroring the same actuator failure across the GM lineup.
Common Symptoms
- Repetitive clicking or knocking behind the dash
- Air only hot or only cold
- Different temperature left vs right vents
- Temperature not matching the setting
- Noise worst at startup or on temp change
How to Fix
Confirm the failing actuator by the clicking location and by commanding temperature sweeps with a scan tool. A relearn/reset can occasionally recover a sticking actuator, but stripped gears require replacing the affected blend-door actuator (an inexpensive part; labor varies with how deep in the dash the actuator sits). Recalibration may be needed after replacement.
Sources for this issue (2)
The plastic gears inside the HVAC air temperature and mode (defrost) actuators strip, so the door stops reaching its commanded position and the actuator motor hunts back and forth. The signature is a rapid clicking or ticking from behind the dash, usually when the temperature or mode is changed or right after startup, together with air that stays cold on one side while the other side blows hot, or airflow stuck on defrost. Owners often mistake it for a low refrigerant charge or a plugged heater core, but the temperature split between left and right vents points at the blend door.
Common Symptoms
- Rapid clicking or ticking behind the dashboard
- Cold air on one side, hot on the other
- Temperature knob has no effect
- Airflow stuck on defrost or won't change mode
- Clicking that starts on ignition on
How to Fix
Confirm by commanding the actuator through a scan tool's HVAC output test and listening for the clicking, or by pulling the actuator and checking whether its output gear teeth are stripped. Replace the affected actuator (air temperature actuator sits behind the left lower instrument panel on the HVAC case; the mode/defrost actuator is a separate unit) and then run the HVAC calibration/recalibration procedure so the module relearns the door end stops - skipping the relearn makes a good new actuator behave like a failed one.
Sources for this issue (2)
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 on a Chevrolet?
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 Chevrolet it is documented across 3 models: Impala, Camaro, Cruze.
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 on a Chevrolet?
Published repair estimates on Chevrolet range from $100 to $500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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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.