B0408 on Chevrolet: How to Diagnose & Fix
Temperature Control (Air Mix) Circuit Malfunction
4 Chevrolet models · 4 documented issues · $100-$1,500 repair
B0408 on Chevrolet by model: symptoms, diagnosis and fix
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)
The A/C compressor clutch on the Malibu fails due to a burned-out clutch coil, worn clutch plate, or bearing failure in the compressor pulley. When the clutch fails, the A/C blows warm air because the compressor cannot engage. A squealing or grinding noise from the serpentine belt area is common before total failure. The compressor clutch can also seize, which will shred the serpentine belt and cause a loss of all belt-driven accessories (alternator, power steering, water pump).
Common Symptoms
- A/C blows warm air
- Squealing or grinding noise from engine belt area
- A/C cycles on and off rapidly
- Burning smell from engine bay when A/C is on
- Serpentine belt shredding or falling off
How to Fix
On some models, the A/C clutch assembly can be replaced independently ($100-$200 for parts, $200-$400 labor) without replacing the entire compressor. If the compressor itself has failed internally (metallic debris in the system), the entire compressor must be replaced along with the receiver/drier, expansion valve, and a system flush ($800-$1,500 total). Always evacuate and recharge the system with the correct amount of R-134a refrigerant after any compressor work.
Sources for this issue (1)
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: 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 (1)
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 4 models: Cruze, Malibu, Impala, Camaro.
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 $1,500 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.
Looking for B0408 on a different make?
View B0408 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.