I have a 2005 v6 mustang with over 100,000 miles. Just recently we had a big leak of radaitor fluid under the car (http://www.allfordmustangs.com/forums/). I drove the car to work one day 75 miles on the freeway and all was fine till i got off and made a few red lights and I could smell the fluid leaking a little again from the cap. Not sure why it was leaking so we started trouble shooting.....
1. - New Radiator cap
2. - New Thermostat
3. - New ECT sensor
4. - Oil Change 5. - Checked all the relays and they are working fine they open and close. With the AC turned on the fan works fine.

now the problems is the fan isn't coming on at low speed or Idle and not sure about high speed....
How do I fix this or trouble shoot more. The temp gauge in the car only goes have way up and no computer errors or warning lights in the dash. I use to hear the fan kick in at a drive thru but I dont hear or see it coming on any more. What else can I do? So Im really feeling that it is the PCM. Is there any test or checks out there that I can do to check if it is the pcm or even test the pcm. I cant find much info on the pcm for 2005+.
Will the pcm loose it programing and need a new reload? Will these after market programs help? Do they reprogram the fans as well and tunning?
switched around the relays of the high and low speed fan with the horn and all seems to be working ok.

I know with a tuner you can adjust the temperatures at which the low/high speed fan turns off/on... but that'll cost you a tune - good excuse for CAI though!

Are you sure the thermostat you put in is rated the same? I would start there first.

yes I am pretty sure the thermostat was the right one. Even so my problem is my low speed and maybe high speed not coming on and the car getting hot enough to run out the radiator cap. If the the thermostat wasnt opening and making the car hot my fan still coming on. Any Ideas on how to check to get my fan to work.

Like another member said, you can use a hand-held tuner to lower the temp for the fan to come on, then it will com on often when the car is at idle. If you don't hear the fan coming on at low speed/idle on a really hot day there may be either a bad connection to the fan. Strange that no codes come up though. 100k miles is pretty impressive though.