When my car reaches a coolant temp of 198F, the high speed fan turns on and it wont stop. It stays at that temp... and the car does not overheat... so i assume that the thermostat is fine. If i use the AC before it reaches 198F, the low speed fan turns on and it turns off when i turn the AC off.. so the fan seems to be working good. I have tried switching relays and nothing. I returned my mustang to stock using a bama tuner (it has camshafts and long tubes) and i set the low speed fan to turn at 212 F and the high speed fan to turn on 232F. After i did the stock tune, it started to do that. A day before i did the stock tune, i changed the coolant in my car. I am guessing that the cylinder head temp sensor or thermostat is bad... any suggestions?