cooling fan not kicking on?

I have a 99 3.4 GT. When I drive on the highway the engine will stay cool. Then when I have to sit in traffic, the car will start to overheat. The fan is not kicking on. I have changed out the thermostat and temperature sensor and it is still doing the same thing. any ideas what this might be

Scan it and see if the fans come on at 222 degrees if so they are ok.

welcome jph. does it accually over heat, like in the red(260+deg)? the fans should come on around 230deg. Their are fuses and relays under the hood on the drivers side(left) fuse box, something that gets over looked occationally. The worsed senario is a blow intake gasket causing the overheating.

I checked all the fuses and relays and they were all fine. It will start to red line, but I have been shutting it off before it does because the coolant will start coming out of the overfill hose. I will usually let it get between 230 and 240 to see if the fan comes on. But no luck...

It may be the fan motor itself. I had a buick that kept overheating and I changed out all the fuses and sensors and it kept overheating but not all the time. I finally pulled the fan motor out and hotwired it and it also tested fine. I started to put it back in and banged it up against the bumper so I checked it again and the motor did not run. I bought one from a parts store and it was less than $30.00. No more problems (with overheating) after that. Just a thought.