* Faulty Blend Door Actuator: The blend door actuator is a small motor that controls the direction of airflow (e.g., face vents, floor vents, defrost). Setting 5 likely uses a specific blend door position that's failing. If the actuator for setting 5 is malfunctioning, it might not move the blend door correctly, leading to no airflow in that specific position. This is the most likely culprit.
* Wiring Issue to Setting 5: There could be a break or short in the wiring specifically associated with setting 5 on the control panel or the wiring harness leading to the blend door actuator. This is less likely than a bad actuator but possible.
* Vacuum Leak (if applicable): Some older climate control systems use vacuum lines to control the blend doors. A leak in the vacuum line related to setting 5 could prevent the correct airflow. Less likely in a 2006, but worth considering.
* Control Panel Issue (less likely): While less probable given 1-4 work, a problem within the control panel itself could be selectively affecting setting 5. This would usually involve a faulty switch or internal circuit.
* Blocked Airflow: While unlikely to affect only one setting, there's a slim chance something is obstructing the airflow path specifically used by setting 5.
Troubleshooting Steps:
1. Check the obvious: Ensure nothing is physically blocking the vents associated with setting 5.
2. Listen carefully: When you select setting 5, do you hear *any* motor sound from the climate control system? If not, the actuator is the prime suspect.
3. Visual Inspection (advanced): If you're comfortable working on your vehicle, you could try locating the blend door actuators (usually under the dashboard). Inspect them for visible damage or signs of binding. This usually requires removing some dashboard components. *This is a challenging task, and unless you're experienced, it's best to take it to a mechanic.*
4. Professional Diagnosis: The best approach is to take your Trailblazer to a qualified mechanic or AC specialist. They have diagnostic tools and the experience to pinpoint the exact problem, whether it's a faulty actuator, wiring, or something else. Describing the specific issue (settings 1-4 work, only 5 fails) will help them focus their efforts.
Replacing a blend door actuator is often a relatively straightforward repair for a mechanic, but the labor to access it can vary depending on your vehicle's specific setup. Don't attempt to diagnose or repair this yourself unless you have significant automotive repair experience.
