Cadillac Blower Motor Dies, AUC Sensor May Be Culprit

Having dealt with a similar climate control issue on my 2008 Cadillac CTS, I can share some relevant experience. While I have some experience with basic car maintenance, I had this specific issue handled by professionals. Initially, the symptoms matched yours, the blower motor stopped working completely after showing intermittent issues with the climate control system. After inspection, it turned out the problem was actually a faulty cable connection to the AUC sensor, not the sensor itself or the blower motor as initially suspected. The repair involved checking the entire auto recirculation system connectivity, and they found a damaged wire harness connecting to the cabin air sensor. The total repair cost was 95€, which included diagnostic time and fixing the connection. However, to provide more specific guidance for your case, could you share: Have you noticed any warning lights on the dashboard? Did the blower motor stop working suddenly or gradually? Does the climate control display still function normally? Can you hear any clicking sounds when changing fan speeds? These details would help determine if your issue matches what I experienced or if it might be something different.

Thank you for checking. Last service was done 2 weeks ago at 150899 KM. The issue started happening after that. To answer your questions: No warning lights on the dash, and the blower motor died gradually over about 3 days. The climate control display works fine but nothing happens when adjusting settings. No clicking sounds when changing speeds. I checked the auto recirculation system fuse and it was good. The odd thing is the AUC sensor seems to be working since the system still switches between fresh and recirculated air automatically. Starting to think it might be the blower motor resistor rather than a sensor issue, but wanted to confirm before buying parts.

Thanks for those additional details. The symptoms you're describing are quite different from what I experienced with my CTS, despite the similar initial concerns about the AUC sensor. While my issue was related to the air conditioning sensor wiring, your case sounds more like a classic blower motor resistor failure. Since you mention the AUC sensor is still functioning correctly with the auto recirculation system working, we can likely rule out sensor-related issues. The gradual failure pattern and lack of clicking sounds strongly point to the blower motor resistor, which is a common failure point in these models. In my case, after fixing the initial HVAC sensor connection issue, I actually had to replace the blower motor resistor about a year later. The symptoms were identical to what youre describing, gradual failure, no clicking, working display but no air flow. The resistor replacement cost me around 85€ for the part and took about an hour of labor. If you're comfortable with basic repairs, its a relatively straightforward DIY job on these models. The resistor is usually located near the blower motor behind the glove box. Before you replace anything though, I would suggest checking the blower motor connector for signs of overheating or corrosion, as this can sometimes cause similar symptoms.

Thanks for the detailed response. You've helped narrow this down considerably. The symptoms do match a blower motor resistor failure more than my initial AUC sensor theory. Since the auto recirculation is still working and switching between modes properly, the cabin air sensor system is probably fine. I actually feel confident about attempting the resistor replacement myself given the location you mentioned. 85€ for the part seems reasonable, and I can save on labor costs. I will definitely check the blower motor connector for heat damage first though, as that could save me from buying unnecessary parts. Good to hear the repair was straightforward on your CTS. Hoping my STS will be similarly accessible. Will update once I check the connector and potentially replace the resistor. If anyone else runs into similar climate control issues, definitely worth checking the resistor before assuming sensor problems.