This question seems to be attracting a lot of views, so I guess a lot of people are having the same problem.
Just to be clear, the problem with MY fans specifically seemed to be caused by my new temp sensor being faulty. This isn't likely to be your problem, but don't rule it out. There are some other great diagnostics suggested below for other possible reasons the fans aren't running.
There was a bigger problem with overheating with this car, even after the fans were running correctly. The stock engine comes with a plastic intake manifold gasket, which is prone to failure. It leaks coolant into the cylinders and causes inefficiency in the cooling system, leading to low coolant and overheating. You can replace this gasket with a metal one, the repair is a little involved but I managed with a Haynes manual. Good luck!
My 2000 Pontiac Grand Am GT (6 cy/3.4L) has been having cooling issues. I started by replacing the thermostat and performing a flush & fill on the radiator. This didn't seem to resolve the issue -- the engine still overheats when at a stop.
This lead me to suspect the fans, so I tested the fans themselves (independent dual electric cooling fans). They do not start when the engine is warm... this is the problem.
I wired each fan directly to the battery, both function correctly. Next, I checked the relays, both tested operational. If I disconnect the temperature sensor, both fans come on immediately. So, I replaced the temperature sensor.
The problem persists; the engine will reach 200, the thermostat opens (determined by watching the temp. gauge), but the fans do not start. If I disconnect the (new!) temperature sensor, both fans start. I have read that turning the air conditioning on should also cause the fans to turn on, this does not happen (with the temp. sensor plugged in), not when then engine is cool or hot.
I am at my wits end as to what to check next. I am prepared instead to hardwire the fans to the ignition switch. Can anyone give me any ideas of any further diagnostic steps before I give up and take the "duct tape solution"? This is my wife's daily driver, so the pressure is on! Thanks in advance.