A4 TDI Cooling System Nightmare: Temperature Swings Alert

I dealt with nearly identical symptoms on my 2003 Audi A6 2.0 TDI. As someone with moderate experience working on cars, I initially thought it was a major cooling system problem too. After checking multiple components of the automotive cooling system, my mechanic discovered it was actually just a blown fuse controlling the engine temperature sensor. The car overheating warnings and gauge fluctuations were false readings caused by the faulty electrical connection, not a real engine overheating issue. The fix was surprisingly simple and cost 52 Euro for diagnosis and fuse replacement. No need for extensive cooling system repairs or radiator work. To better assist with your case, could you share: Any error codes from the check engine light, Whether the temperature fluctuations happen more during city driving or highway speeds, If you notice any coolant loss, Whether the heating system works normally These details would help determine if your issue matches what I experienced or if you might be dealing with a different problem. Always worth checking the simple things like fuses before diving into major cooling system repairs. Keep monitoring your coolant level and watch for any actual overheating while diagnosing the issue.

Latest service was at 69100 KM, just a few weeks ago. Thanks for the detailed response about the fuse. I pulled the error codes: P0128 and P0115. The temperature swings happen mostly during city driving, and I have not noticed any coolant loss. The heating works fine except it takes forever to get warm air flowing. After checking the fuse box, all fuses related to thermal management look good. I am starting to think this might be related to restricted airflow, since the radiator grille area has quite a bit of debris buildup that could impact the cooling system performance. Would cleaning the radiator area be a good first step before exploring other potential causes of these engine overheating symptoms?

Thanks for providing those additional details. The error codes P0128 (coolant thermostat rationality) and P0115 (engine coolant temperature sensor circuit) combined with the slow warmup actually point more toward a failing thermostat than airflow issues. While cleaning debris from the radiator grille is good maintenance practice, the symptoms you describe, especially the slow warmup time, typically indicate a thermostat stuck open rather than a radiator blockage. When stuck open, the thermostat continuously circulates coolant through the radiator, preventing the engine from reaching proper operating temperature. In my A6, I initially cleaned the radiator grille area too, but it made no difference to the warming issue. The thermostat replacement solved all symptoms: Normal temperature readings, Faster warmup times, No more check engine light, Proper heat output The repair cost about 150 Euro including parts and labor. If you want to verify before replacing, try monitoring the coolant temperature with an OBD scanner during a cold start, a properly functioning thermostat should maintain steady temperatures once warmed up. The automotive cooling system is pretty sophisticated in these cars, and while keeping the radiator grille clean helps overall cooling performance, your symptoms strongly align with a thermostat malfunction.

Thanks for the input. The thermostat explanation makes a lot of sense given my symptoms. After researching the P0128 and P0115 codes more carefully, I agree that my initial suspicion about restricted airflow and radiator blockage might have been off target. Going to schedule an appointment to have the thermostat replaced since all signs point to that being the core issue affecting my cooling system performance. The slow warmup and temperature fluctuations match perfectly with a stuck-open thermostat causing improper thermal management. Much more confident now about avoiding unnecessary repairs. Will report back on whether the thermostat replacement resolves the car overheating warnings and gauge behavior. Really appreciate the detailed technical advice, saved me from focusing on the wrong components.