B0408 on Cadillac: How to Diagnose & Fix
Temperature Control (Air Mix) Circuit Malfunction
1 Cadillac model · 1 documented issue · $120-$450 repair
B0408 on Cadillac by model: symptoms, diagnosis and fix
Two related HVAC complaints are common on the XTS: a repetitive clicking/knocking from behind the dash caused by a failing blend-door (temperature) actuator that can't reach its calibration stop, and loss of fan-speed control or no heat from a failed blower motor resistor/module — when the resistor fails the fan often works only on the highest setting. Owners report inconsistent temperature (one side hot/one cold) with the actuator fault and a dead or single-speed blower with the resistor fault.
Common Symptoms
- Repetitive clicking/ticking behind dash
- One side blows hot, other cold
- Blower only works on high speed
- No fan / no air from vents
- Temperature won't change
- No heat or no cold air
How to Fix
For the clicking: identify the failing blend-door actuator (listen/feel behind the dash while cycling temperature), replace the actuator and re-run the HVAC calibration/relearn. For no/limited fan: test the blower motor resistor/control module and the blower motor itself; replace the failed resistor module (and motor if it's drawing high current). Clear any HVAC actuator codes 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 Cadillac?
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 Cadillac it is documented across 1 model: XTS.
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 Cadillac?
Published repair estimates on Cadillac range from $120 to $450 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.