Although rare, it is possible that the fan motor is bad. I don't know if the connectors are the same or if there is only one connector for the fans. My point is try to switch the connectors and turn the a/c on. One fan runs off the coolant temperature sensor and the other off the a/c hen the compressor is running. It basically senses a load on the engine and engages the fan. The one that works has power to it so you know if it does not run the other fan, the problem is the fan. If you have the V6, it may just not be getting hot enough for the fan to kick on. If you have the 5.7L, I would definitely confirm the fan is good because those engines are designed to run hot and they get up to running temperature very quick. Please rate me on this answer.