Audi Q5 Thermostat Mystery: Hot and Cold Issues Fixed

I had a very similar issue with my 2010 Audi A4 2.0T. Having some experience with cooling system repairs, I recognized these contradicting temperature symptoms right away. The map thermostat was indeed the root cause. The temperature sensor within the thermostat housing was malfunctioning, causing incorrect readings and improper temperature control. This component regulates coolant flow based on engine temperature, and when it fails, you get exactly these symptoms, both slow warm-up and occasional overheating. My repair experience: After confirming the coolant system was properly filled and the water pump was working correctly, the workshop diagnosed the faulty thermostat. The total repair cost was 350 Euro, including parts and labor. The fix completely resolved both temperature issues. Before suggesting specific solutions, could you share: Are you getting any error codes? Does the temperature gauge show erratic behavior? Have you noticed any changes in fuel consumption? How many miles/kilometers are on your engine? These details would help confirm if your issue matches what I experienced with my A4.

Thanks for sharing those details. Based on the last service at 112796 KM, I can confirm we are dealing with a similar thermostat housing issue. The cooling fan on my Q5 was also running irregularly which turned out to be connected to the faulty temperature sensor readings. The repair resolved all temperature-related problems and cost about 400 Euro total. Make sure they check the entire coolant system during the fix, as these issues sometimes come together with worn coolant lines, especially at our mileage point. The engine overheating symptoms disappeared completely after the thermostat replacement. A proper diagnosis should include checking the temperature sensor separately before replacing the whole thermostat housing unit. Has your cooling fan behavior been normal or do you notice irregular running patterns?

Based on my experience with the A4, the cooling fan behavior is definitely tied to the temperature sensor issues. After monitoring my car post-repair, the cooling fan now operates in a much more predictable pattern. Before the fix, my cooling fan would out of nowhere activate at cold starts or run continuously even when the engine was barely warm. The faulty temperature sensor in the thermostat housing was sending incorrect signals to the temperature control module, causing these erratic fan patterns. When I had the thermostat replaced, the technician performed a complete cooling system pressure test and found that the irregular fan operation was solely due to the bad sensor readings. No issues with the coolant lines in my case, though it was smart to have them checked given the mileage. The repair completely normalized the cooling fan behavior along with fixing the temperature regulation. Now the fan only kicks in when actually needed, typically at higher engine loads or when idling on warmer days. This more predictable pattern confirms the thermostat housing replacement was the right fix for both our cars. Worth noting that the temperature sensor replacement alone might not always solve the issue if the thermostat mechanism itself is also worn, which is why replacing the complete unit often makes more sense.

Thanks for the thorough follow-up. I have noticed my cooling fan is actually acting up similarly, running at unusual times during cold starts and sometimes staying on longer than it should. Your experience confirms my suspicion about the thermostat housing being the main problem. The contradicting symptoms of engine overheating followed by slow warm-up cycles make more sense now. Temperature control issues seem to trigger these irregular cooling fan patterns. I will have the complete thermostat unit replaced rather than just the sensor since both parts have the same mileage. One remaining question, did you notice any improvement in fuel consumption after your repair? Since the temperature control is now working properly, I wonder if this might also affect efficiency. My fuel economy has been slightly worse lately, which could be related to the cooling system not operating at optimal temperatures.