Ok I have a 2007 Mercury Grand Marquis it was waiting on line at a drive thru and my car overheated and the engine light came on. I checked the code it was P1299. I called my mechanic he came over and he replaced the thermostat because it was stuck opened and the sensor that's behind the alternator which the code refers to. The engine light and code was gone but then the cooling fan wouldn't come on sonow he replaced the fan motor and module as soon as he attached everything the fan came on without the car turned on. He checked the fuses and in the process of looking for shorts in wiring he had to disconnect the fan because it won't stop running. Can you give me anything else he can possibly check he thinks it could be the PCM but wants to make sure it's not something else first.

 

Any codes at this time? The fan probably caused your overheating. A stuck open thermostat will not. I believe you either have a short in the fan wiring or a bad (stuck) fan relay. PCM is possible but the least likely cause for your fan problem.

 

there isn't anymore codes we changed the fan motor and module along with changing the cylinder head temperature sensor and thermostat. After we changed the cylinder head temperature sensor the fan stopped working so replaced the fan moror and module and as soon as he plugged it in the fan went on even without the car being on and it won't stop. A few people are suggesting either a defective cylinder head temperature sensor or the wiring. Does that make sense to you?

 

From what you say, it appears that you need a new mechanic.
Your first clue was that a "stuck open" thermostat will NOT cause overheating.

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