What I learned about no heat

First, make a distinction between no heat- as in zero ...nada and weak heat. If you have some or weak heat...this isn't your issue....but if you have zero heat...this might help....read on.

The "crisis" situation is the heater core...calm down and check something else first.

The next "crisis" is the blender door spine is broken...calm down.

What I found on mine was the blender door actuator failed. If you look under the dash just to the right of the center console you will find that actuator. When you turn the temp blender from cool to warm that actuator will turn and will move your blender door. My actuator wasn't turning....so my blender door was stuck in the cool position...thus zero heat.

The actuator is not expensive and is accessible without removing any dash components. It's a pain to access...get a small wratchet with a u-joint connector. I think it's 8mm bolts...three of them.

When you remove it see if you can turn your blender door. Should turn with fingers. Then, plug in your replacement actuator...and turn your temp dial to make sure the new one turns. Then, turn the dial all the way to one side....and manually turn the blender door to match the fitting...that way you will know it's aligned and you won't over turn that blender door and snap it.

Screw it back on....and hopefully heat is back.

Since you did not indicate the model year, does this pertain to the 1996 and older XJs, the 1997+ models or all of the above?
Just wondering.

Sorry. This is for a 2001, so it would apply to a 1997 and above.
The earlier model have a cable pulley system of some sort.