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I have a 2000 Nissan Altima that has an overheating problem.  The radiator fans do not engage nor does the AC compressor.  Things I have done to try and eliminate the problem are: Changed the thermostat sensor, replaced the thermostat, checked the fuses to include relay, hooked the fans to the battery and they kicked on.  I also ,of course changed the radiator fluid and when doing so reved the engine and radiator fluid splashed out (figuring that would verify that the water pump was working properly) and there is no weeping from the water pump hole.  The heater is working properly blowing hot air.  The automotive book suggets that when the AC and the fans do not kick on it could be the on board computer.  But before I shell out $300-$500 I wanted to see what you guys thought.
Thanks any help would be greatly appreciated!  

if you know how to test things your self...

buy a scan tool...  one that displays live data...  harbor freight has one..   98614...  it will access some of your systems.. yes.. you can see when the ecm calls for the cooling fan operation..

there is a better scan tool from the same manufacturer.. Hf did sell them but sold out.. it is online at amazon....   JP702 or JP703... it will access most of your cars systems..  and is easy to use...      

you will also need professional repair information..

alldatadiy.com....   autozone sells a single subscription disc in their stores..

eautorepair.net..   this is mitchells  online site..

these 2 are the same data that professional techs have access to for much more..


i looked at the free autozone site.. they don't list the cooling fan controls for your car...    

so i don't know what controls your radiator cooling fan.. a thermal fan switch or a temp sending unit that tells the ecm how hot the motor is...

perhaps one of the others who read this... i cannot answer every post.. but i try..