VW Phaeton HVAC Failure: Defrost Flap Malfunction Fix

I had a very similar issue with my 2006 VW Touareg diesel last winter. Having worked on various climate control systems, I have some experience with these types of problems. The symptoms you describe match exactly what I encountered, inconsistent temperature regulation and poor heating performance. The dashboard panel showed error codes related to the air distribution system. My ventilation system was also struggling with proper airflow distribution. After checking various components, my workshop found that the wiring harness connecting to the climate control module had deteriorated. Some pins in the connector were corroded, causing intermittent communication issues. The repair involved replacing the damaged wiring and connector, which cost 95€ in total. However, before assuming its the same issue, it would help to know: Which specific error code is showing up? Does the problem occur only when the engine is cold or all the time? Are you hearing any unusual sounds from the dashboard area when adjusting temperature? Have you noticed any changes in the air conditioning performance as well? While the blower motor might not be the primary issue, these heating problems often involve multiple components in the air distribution system. Knowing these details would help narrow down the exact cause in your case.

Current mileage: 104449km Last service: Annual inspection completed last month Thank you for the detailed response. The error code showing is P0646. The heating problems occur consistently regardless of engine temperature. No unusual sounds from the dashboard when adjusting settings. Air conditioning works normally in summer. After your suggestion about checking connections, I inspected the wiring around the heater core area and noticed some wear on the insulation. The air circulation seems most restricted on the driver side vents. Would replacing just the wiring harness be worth trying first before looking at more expensive components?

Thanks for those additional details about your Phaeton. Since we have similar models and issues, let me share my further experience. The P0646 code and those wear patterns you spotted in the wiring are very telling. In my case, the ventilation system issues also started with the driver side showing reduced airflow. When I first replaced just the wiring harness, it temporarily improved the temperature regulation, but the problem returned after a few weeks. It turned out the failing wiring had actually caused damage to the air distribution flap actuator. The inconsistent voltage damaged the motor that controls the flap position. My complete fix required: 1. New wiring harness (120€) 2. Flap actuator replacement (215€) 3. Recalibration of the climate control module The windshield fogging you mentioned originally was also resolved after fixing these components. Looking at your symptoms and mileage, I strongly suspect you are facing the same underlying issue. I would recommend replacing both parts together rather than just the wiring. While it costs more upfront, it prevents potential damage to the new wiring from a faulty actuator and saves labor costs of doing the job twice. The blower motor can remain untouched since your AC performance is normal.

The feedback about combined replacement makes a lot of sense for my Phaeton situation. Given the similar model year and fault symptoms, I think proceeding with both the wiring harness and flap actuator replacement is the most logical approach. The total cost around 335€ seems reasonable compared to risking repeated repairs. My next steps will be: 1. Scheduling the repair for both components 2. Requesting the climate control module recalibration 3. Having them verify the air circulation is properly balanced after repair Really appreciate the detailed response and repair cost breakdown. This should resolve both the temperature regulation issues and the windshield fogging problems. Once the ventilation system is working correctly again, I will report back on the results in case other Phaeton owners encounter similar heating problems.