Running warmer then it should be. No leaks, the cove said it was was not opening it soon enough
My car has 1139000 miles.
My car has an automatic transmission.
If you have a code that says the thermostat is opening to late, I would just replace the thermostat. Codes indicating thermostat problems are usually accurate. I would only recommend being sure the cooling system is completely full before you begin replacing the thermostat. You indicate there aren't any leaks, so it is likely to be full, but it is possible to loose coolant slow enough that you won't notice any leaks.
Checking the cooling system should be done when the motor is cold. Checking the coolant level on a hot vehicle is very dangerous. So remove the radiator cap, not just the coolant overflow tank cap. The most accurate way to see if the coolant is full is by removing the radiator cap first thing in the morning. If you do find the coolant is low, still replace the thermostat. Thermostats are known to fail as a result of other cooling system issues.
Some other possibilities are a plugged radiator, inside it and outside. I wouldn't expect a plugged radiator if the coolant is clean, but it would be highly suspect if the coolant is in poor condition. Also, the exterior fins of the radiator can become clogged as well. But I wouldn't expect either of these to be the main issue with the code that has been set. Although, these two points of clogging are very common sources of a car running warmer than normal, but without the thermostat code.
If you decide to get this fixed, consider YourMechanic, as a certified technician can diagnose your overheating issue and make the necessary repairs.
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