If everything under the hood seems fine, I wouldn't worry too much about it. Might just be a loose connection somewhere.

 

If 91 has 02 sensors it could because of that. Going higher alitudes will affect oxgen levels which will cause it to read little different.

 

I had a phantom light on my 95. I had just finished some suspension work (nothing that would seemingly trigger the old school engine code) and I was driving the 6 mile trip from where I was working on the Jeep to my house and the light came on pretty early on the drive. I got home and went to cycle the key to get the code and - poof - nothing. It has never come back.

 

I don't think any suspension work is realated. I wouldn't worry too much. With the pre ob2. Computer s alot is in peramiters. So if somthsome was out check engine light would come on. Then if it fixed itself it would be off on next key cycle. Like on ob2 computers there' pending codes then hard codes. Pending will fix it self if not then set it as a hard codes. Same idea

 

Hahaha - I'm not worried. I was just putting it out there so the O.P. knows he isn't the only one that's had an ODB1 phantom code. It's been weeks since the code and my Jeep runs well and continues to get about 19mpg on average.
It passed inspection as well since the phantom code, but they don't check emissions (or OBD1 I believe) since the vehicle is more than 15 years old.

 

Yea emissions blow lucky my 98 doesn't have to pass it in NH either.

 

Nice - where in NH are you?

 

Southern near Hinsdale NH. Near Keene NH. Are u from NH

 

Groton MA is a couple towns from the NH border, near Nashua.

However my wife and I go camping every year out in your area - Swanzey on the Ashuelot river. We took the XJ this year and I noticed there were a lot of XJ's around there. In fact the guy who owns the campground we go to has one.