2002 Grande Cherokee Overheating

Here's what I got: Basically the Jeep overheats when sitting idle. Once you start moving the temp will come down. I noticed the radiator fan never came on so I replaced it, but it still isn't coming on. Expensive wiff, but I still need to find out why the fan isn't coming on (relay?). I guess I'm looking for ways to better diagnose the issue without throwing parts at it.

The radiator looks like it's in great shape. I was thinking about going after the thermostat next, but it seems like the temp wouldn't go back down when you started moving if it was the thermostat. The belt also looks like it's in good shape so I don't think its slipping. I know I've also replaced the radiator cap on other vehicles, but I can't remember how a bunk cap manifested itself in a problem.

replace the relay, shpuld be under passenger headlamp, this controls the fan.

I did end up changing the sensor and it fixed the problem.

Thanks to all that make posts!