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AC or Engine ? If engine it may be the engine coolant temp. Fan switch. Its either located on. Lower radiator or thermost housing. If you can unplug the wire at sensor and ground the wire. If fan runs you need to replace sensor.
It could well be a faulty thermostat- test or replace same.
Could also be a blocked radiator core, - check coolant or airflow is not restricted,
Ensure any radiator leaks are repaired,
Ensure coolant level is correct, without airlocks - bleed as necessary on re-fill,
Check operation of cooling fan - ensure fan runs at appropriate engine temperature - if not check sensor switch and associated relay operation.
Thermostat is located at the end of top radiator hose where it connects to engine housing.
Check the wiring from the relay to the fan motor, and check the wiring from the fuse block. Basically check all wires that supply power to the fan motor, use a volt meter. Fan Relay (behind headlight, under battery) on the 2001 GC 4.0L are a piece of ****, I change mine almost yearly. The problem is their location, a high heat area prone to the elements. I understand that 2002 and newer, the location was changed and the problem solved. Relay might close the circuit when cold or at reasonable temps, but might open a circuit when it heats up, causing the problem.
Have you tested the fan, use a set of clip leads and supply 12 volts of battery power to the fan to see if it runs, another test is to disconnect the engine coolant temp fan switch, this sends the system into default and the fan will run.
Had this problem and one thing that even Chilton manuals only show in fine print sorta say is that Jeep cherokee has a relay sensor for the coolant fan usualy right behind the passenger or driver side headlight you may have to remove the bumper also its rather easy.Its a liitle black box thats screwd down on the panel just unscrew it unplug it replace mine ran me around $60.00.Before I learned about this after replacing the thermostate and just about to rip out the water pump my buddy told me about the sensor after replacing over a year ago no problems.