Can be leaking intake manifold gaskets, which can sometimes be fixed by using Dexcool coolant. Can also be a head gasket.
Possible causes: Insufficient/no coolant in the system; insufficient antifreeze in the coolant and the coolant is frozen: the water pump is faulty and is not circulating the coolant; the belt that drives the water pump is loose or missing.
This is a diagnostic process. If the coolant isn't low and the fan will run when the AC is turned on it will need a Mechanic to Diagnose.
Dexcool
Check the fan control
The Orange Dexcool
DEX Cool
go to auto zone they will do it for you
If there is coolant coming from it and it isn't a hose.
A 2001uses R134a refrigerant.
the simple answer is the coolant fluid - make sure it's full where it's marked, after driving it ten miles. the bigger problem is unlikely if it runs alright, so try the first. you might just need to flush and fill the coolant too..
Check for a sickly sweet smell. The heater core is probably leaking coolant into the floor. The core is not repairable but can be replaced.
get a new temperature sensor.