1997 Nissan Altima Engine Fans

Hi,

Background:

My car was leaking radiator fluid. I checked the hoses and they were not leaking nor was there leaking around the clamps. So it was either the radiator that I replaced the February along with new hoses (they were old) and clamps.

I took the car to Firestone and they did a pressure
check. The water pump was fine but the thermostat was leaking. So we replaced it 2 weeks ago.

Question:

Ever since we replaced the thermostat, often when I park the car, the fans would go on for 15 seconds. Sometimes the fans go on briefly when I am at a stop light .

The reason being I do not remember the fans going on unless it is summer. I live in Phx, AZ and the temperature is in the 70's during the day and drops to the low 60's at night.

I would think with these temperatures the car fan would not need to go on.

Is this normal?? Is the thermostat not working (opening) right??? This all started right after we replaced the thermostat.

Thank you.

updated 12-2-08
Thank you legitimate007 for the quick and detailed response!

I tried giving feedback but when I submitted the feedback I got a error
message saying "you can't give points more than you have!"

I also did replace the coolant. If it is not too costly, I will replace the
engine coolant temperature sensor in March when I do my yearly tune up.

Thanks again