VW Lupo HVAC Failure: Temperature Sensor Alert

Had almost the exact same issue with my 2002 VW Polo diesel last summer. Based on my experience with car repairs and HVAC systems, this sounds very familiar. Initially thought it was just a sensor malfunction, but after getting it properly diagnosed, the main control unit for the AC was completely dead. The sensor calibration process would not even start due to the faulty control module. The symptoms matched yours exactly, no AC function, error messages on the cluster, and diagnostic codes pointing to the HVAC system. Before going to the workshop, I checked for any obvious coolant leaks and basic electrical connections, but found nothing visible. The repair involved replacing the entire AC control unit. Total cost was 980€, including parts, labor, and a full system test. The workshop also performed a complete vehicle maintenance check while they had it apart, which was helpful in preventing future issues. Since the fix, everything has worked perfectly with no temperature control problems or error messages. Before suggesting more specific solutions, could you share: Current mileage on your Lupo, Any recent work done on the AC system, Whether you notice any unusual sounds when trying to activate the AC, If the blower motor still works on different speed settings This information would help determine if your issue is identical to what I experienced.

Last service was actually done at 34000km, and after checking the service records and fault codes again, I noticed the temperature readings were fluctuating before the system failed completely. The blower motor works fine at all speeds and no unusual sounds occur when pressing the AC button. The coolant level remains stable and proper service records show no previous HVAC system repairs. Worth noting that the sensor malfunction warning started appearing after a particularly hot day when the AC was running continuously. The heat exchanger seems to be functioning, as the engine temperature stays normal during operation. Perhaps we should also consider checking the wiring harness between the control unit and the temperature sensor before replacing expensive components.

Since we dealt with similar HVAC system failures on our VWs, let me share what I discovered after my initial repair experience. Your symptoms show some key differences from what I encountered. The stable coolant level and normal engine temperature readings suggest your heat exchanger is functioning correctly. This actually points away from my initial control unit diagnosis. The fluctuating temperature readings before total failure strongly indicate a temperature sensor issue, rather than the complete control unit failure I experienced. When my control unit failed, it was sudden and complete, without prior warning signs. The wiring harness check is a smart approach, wish I had considered this before spending 980€ on a new control unit. During subsequent maintenance, my mechanic showed me how temperature sensor connections can deteriorate over time, causing intermittent readings. For your case, I recommend: 1. Testing the wiring harness connectivity 2. Checking the temperature sensor resistance values 3. Inspecting the sensor mount for proper contact A temperature sensor replacement typically costs between 150-200€ including labor if that turns out to be the issue. Much better than a full control unit replacement like I needed. This aligns more closely with your symptoms, especially considering the gradual onset after extended AC use during hot weather.

Thanks for the detailed comparison to your AC repair experience. The fluctuating temperature readings you mentioned definitely match what I am seeing. Good point about checking the wiring harness first, makes sense to verify the connections before replacing the temperature sensor. After getting advice here and researching more about the cooling system components, I will start with a thorough inspection of the wiring and sensor mount. A 150-200€ repair cost for the sensor replacement sounds much more reasonable than replacing the entire control unit. The HVAC system behavior on my Lupo matches the temperature sensor failure pattern rather than a total control unit issue. Since the heat exchanger and coolant levels are good, I will focus troubleshooting on the sensor and its connections. Will make sure to get proper sensor calibration done if replacement is needed. Will update once I get it checked out. Hoping the wiring inspection reveals the root cause without requiring major component replacement.