Audi S5's Bizarre Hot/Cold Temperature Symptoms

I faced nearly identical symptoms with my 2011 Audi S4, which pointed directly to a defective map thermostat. The contradicting temperature behavior you describe is a classic sign of thermostat failure in these engines. The coolant system would struggle to maintain proper temperature control, sometimes showing engine overheating warnings while other times taking unusually long to reach operating temperature. During diagnosis, it was discovered the thermostat was sticking intermittently, causing irregular coolant flow. The repair involved replacing the entire thermostat housing assembly since the electronic map control unit is integrated into it. This is a known weak point in these engines and definitely requires professional attention. While not catastrophically urgent, postponing the repair risks more severe engine overheating issues and potential damage to other components. The job took about 4 hours at the shop, requiring draining the coolant system and accessing several hard-to-reach mounting points. After replacement, the temperature regulation returned to normal, with consistent warm-up times and no more overheating warnings. Best to address this sooner rather than later, as continued temperature fluctuations can impact engine longevity and performance.

Thanks for sharing your experience! Your symptoms match exactly what I went through. I also had to get the thermostat housing replaced, which solved the engine overheating problems completely. The water pump looked fine during inspection, but the mechanic recommended replacing it preventively since they had already drained the coolant system. Do you remember roughly how much you paid for the repair? Also curious if youve had any other cooling-related issues since the fix?

After having the thermostat cooling circuit motor replaced, my S4 has been running perfectly with no temperature issues. The repair cost was 350 Euro for the defective map thermostat, which is typical for this type of repair. The cooling system has been working flawlessly since the fix, no more irregular temperature readings or engine overheating warnings. I decided against the preventive water pump replacement since mine showed no signs of wear. The cooling fan and other components were also in good condition. If your mechanic spotted potential issues with the water pump, it might be worth considering while the coolant system is already drained, but in my case, it wasnt necessary. Its been two years since the repair, and the temperature regulation has remained consistent. The engine warms up normally now and maintains proper operating temperature even during long drives. This repair seems to be a permanent solution for the thermostat housing issues common in these engines.

Thanks everyone for the great advice. I finally got the repair done last week, and wanted to share how it went. Took it to a different shop this time that specializes in German cars. They confirmed the thermostat housing was failing and replaced it along with a temperature sensor that was also showing signs of wear. The total came to 480 Euro for parts and labor. While not cheap, the cooling fan is working properly now and the engine temperature stays perfectly steady. No more wild temperature swings or warning lights. The warm-up time is now normal too, takes about 5 minutes to reach operating temperature. What really impressed me was how thorough they were in checking the entire coolant system while they had it apart. Found a small leak at a hose connection that I hadnt even noticed, and fixed that too at no extra charge. Much happier with this shop than my previous mechanic. The car runs like new again and I have peace of mind knowing the cooling system is properly sorted.