My 03 crown vic it's overheating!

Maybe the cooling fans are not turning on except when the AC is turned on.

What part would fix it? Please and thank you

I would have to find a wiring diagram of the cooling fan controls but perhaps it is the temperature sensor. When it overheats are the cooling fans working?

Would you replace thermostat first or not

Yes the fan comes on. But it doesn't cool off until a.c. fan is on.

Coolant temp switch/sensor as with the A/C on the computer by-passes temp semsor to run the electric fan. May can see if it has a bad connection or test or just replace it.

Go ahead but I do not think that is the problem. You may want to flush the cooling system and install new coolant, hoses and thermostat while you are at it. Do check to see if the cooling fan is working when the engine is hot.

If thermostat is bad turning a/c will not make thermostat work. Is it good coolant or just water?

It has coolant in it. It's strange that air conditioning makes it cool off. Thanks for your time

Yea most times this indicates that the coolant temp sensor is failing and not doing its job to turn the fan on but if a/c is on the computer overides the temp switch to turn fan on while a/c is on, so it then cools off, but you may have another issue, like dirty radiator or weal hoses.

While it is hot and a/c off the fan should be running, if not replace the temp sensor switch or fix wire harness or connector or computer. Is it running hot and loosing power too? Check for clogged exhaust Cat. Is there a check engine light on, if so have the codes read.

It's not losing any power. I'm gonna start with sensor and thermostat. Flush radiator. Then I will let you know from there.

Fan kicks on when either heat or a.c. is engaged. Replaced coolant temp sensor to no avail.