I might look at a XJ that I have a weird feeling about even though I haven't seen it yet, something just doesn't feel right for some reason. If I have any doubt I don't have a problem walking away but I'd like to go into it with as much knowledge as possible.
I've never actually had a CEL come on any any rig I've owned so I'm pretty clueless and lucky I guess. If I were to test drive an XJ with OBD1 or 2 and stop by an Autozone and have them plug in a scan tool could anything come up even if there is no light on?

 

stored codes and freeze data(if scan tool is capable) would come up if its an obd II , not sure about obdI never worked with them.

 

Before I buy a car I make sure the check engine comes on if it has one. Someone can cover the light or take the light off. But just take it to autozone and have the codes checked. I also check the compression on the engine. So have that checked too.