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96 Jag check engine light

My check engine light is on for the third time when they put the cmptr tester on they don’t get a clear cut answer they shut it off and about a week later its back it seems to come on when I start the car after an overnight of sitting. A mechanic suggested using an additive in the gas tank and that kept it off the longest. I forget the name of it or I would try it again. Does anyone have any inexpensive ideas?

The computer doesn’t ever give a clear cut answer. It just points to certain areas that serve as a starting point for diagnosis of a problem. Not-so-great mechanics don’t bother with the diagnosis - they shrug a lot, guess, and throw parts at things. Don’t let anyone start doing that.

Look at your invoice or call your mechanic and ask for the actual codes that the computer spit out. As a '96 car I’m pretty sure that these will be OBD-II codes so you can also go to auto parts stores who will sometimes read them for free - try a big chain like Advance or Autozone. (Its possible your system is OBD-I - '96 was around the transition time - in which case they won’t be able to do it).

Figure out what the actual codes are - don’t listen to what anyone says about them - and then post the codes up here. They will normally be a letter followed by a 4 digit number (e.g. P0123). Also post as much info as you can about the car (something about maintenance history, how its been running, if its been doing anything funny, etc.)