Which Temp sender to change??

96 XJ 4.0 auto with AC.

A while back a few times I pulled the codes 12, 21, 55 coolant temp sensor I believe. I got the part but the check engine light wasn't coming on and I was too lazy to put it in. When reading a post somewhere about which coolant temp sensor (the manifold one or the thermostat housing one) was hooked to the computer, I got kinda confused I think. Now I'm not sure which one that this code is indicating is out of range.

So, which one should I change??

Bump, anyone??

The sender for the guage or light is located in the back of the head.(temp)
The sensor in the thermosat housing is for the ECU(computor),and aux fan.
The MAT sensor is in the intake manifold,
The #21 code is for the 02 sensor.
Wayne