Sounds like it could be the IAC valve (idle air control) valve. When they start acting up they won't throw a code most of the time. If it is the valve it's a real pain to change, it's at the back of the intake, passenger side.
If the light is on and there are no codes, you have a computer failure.
if it is OBDII hook it up to the tester which can be done for free at advanced auto parts. Advance auto parts in my area will no longer clear codes for you.
Auto Zone will check your engine codes for FREE. After finding out what cause your engine light to come on. Then the problem can be resolve with out guessing.
Auto Zone will read your engine lights codes for FREE.
If the check engine light is on for a 95 Lincoln town car, check the computer codes. a problem could cause low fuel economy, or cause a part failure over time.
There are pages and pages of things that can turn on those lights, you need to have it checked for codes with a scan tool and can diagnose from there.
You cannot adjust the idle in a 1998 Cadillac Catera. It is computer controlled. If the idle is not at 750 rpms then there is another problem with the engine. Check for codes with an OBD2 tester.
First the light has to be connected to a tester to erase the fault codes. Then it should be programmed to get the new codes and it works.
one way is to take the ground cable loose from the battery. leave it off for a few mins. or if you have a code tester plug it in and hit erase codes.
Check all the basics, plug wires, codes(check engine light), Vacuum tubing, etc...could be alot of things, but probably not in the O2 system...also, your local parts store should be able to loan a tester for the Check engine codes...:)
Hard to tell from here if the engine has a shutter or the transmission has a shutter but if the check engine light is on you could start with having the ECM scanned for codes.
It could be a nubmer of reasons, you may need to get a code tester to see what codes may be showing up to determine why the light comes on. Most likely a bad sensor?