Audi A4 Engine Temp Swings: Stuck Thermostat Alert

Had the exact same symptoms on my 2019 Audi A5 last winter. The engine temperature was unstable, sometimes too hot, sometimes staying cold too long. After thorough diagnostics, the issue was traced to a failing thermostat housing assembly. The thermostat was stuck partially open, causing inconsistent coolant flow through the cooling system. This explained both the overheating and the slow warm-up cycles. The water pump and temperature sensor tested fine, but the thermostat housing had developed hairline cracks, leading to a minor coolant leak. The repair involved replacing the complete thermostat housing unit with integrated sensors. This was not a quick fix situation, the entire front end needed to be accessed to properly replace the component. While serious enough to require immediate attention, it was not a catastrophic issue when caught early. The symptoms you describe are textbook indicators of thermostat failure. Would strongly recommend having the thermostat housing checked by a qualified technician, as continuing to drive with these symptoms could lead to more severe engine damage.

Thanks for sharing your experience. I had similar thermostat issues with my Audi A4 last summer. Can you tell me the repair costs in your case? Also curious if you encountered any other cooling system problems since the fix. My mechanic mentioned the water pump might need attention soon too, so wondering if that came up during your repair.

After getting the thermostat housing assembly replaced, my Audi A5 has been running perfectly with no temperature control issues. The repair cost was 350 Euro for fixing the defective map thermostat, which resolved all the erratic temperature readings I was experiencing. The cooling system has been working flawlessly since the fix, no more engine overheating or slow warm-up problems. My mechanic did a thorough inspection of the water pump during the repair, but it was in good condition and didn't require replacement. Regular coolant system checks during maintenance have shown everything is operating as it should. The temperature sensor readings have remained stable, confirming that replacing the thermostat housing assembly was the right diagnosis. If your mechanic suggests preventive water pump maintenance, it might be worth considering, but in my case, it wasn't necessary as part of the thermostat repair.

Just had the repair done last week and wanted to update everyone. After getting a second opinion, I confirmed it was indeed the thermostat causing my temperature control problems. The new shop found small cracks in the thermostat housing that were causing a minor coolant leak. The total repair came to 380 Euro including parts and labor. They had to remove several components to access the thermostat housing, but the job was completed in one day. The mechanic did a pressure test on the entire cooling system and found the water pump was actually fine, saving me from an unnecessary repair. Been driving for a week now and the engine temperature stays perfectly stable, no more overheating issues or slow warm-ups. The cooling system is working exactly as it should. Really glad I got this fixed before it led to more serious engine damage.