I have a '95 Cavalier, the check engine light comes on from time to time...but not regularly...?

Question

Does it have to be on in order for it to be tested...and if so what might it be?

Answer 1

no, it doesnt have to be taken in to read the codes. 93-96 (some early model 97s as well) are OBD-1 Engines (On Board Diagnostics). 96-current are OBD-2 and a computer is required to get the codes. What you need to do is find the diagnostics box under the hood. Its the size of a d battery, and says "diagnostics" on the cover. Open it up, and there are 10 holes. arranged like so
A B C D E
K J H G F
With the car shut off, and the key out of the ignition stick a paper clip in too a and b and this will 'short out' the diagnostics (it wont damage the car at all, and can be repeated countless times without worry). (A will be Ground, B will be ECU)
Then turn your car on, dont start it, just turn it to "ACC". THen your check engine light will flash. You need to count the number of flashes.
SO it will be like
Flash Flash Flash....(short pause).... Flash Flash....
So if you get 10 flashes in a row, then a short pause, and two more flashes, it refers to code '12'
If you get 3 flashes, a short pause, and 6 more flashes, it refers to code 36.
If you get 3 flashes, a short pause, 6 more flashes, then a long pause, 3 more flashes, a short pause, 2 more flashes, that refers to two different engine fault codes, 36 and 32.
Here is a list of what the codes mean:
http://everything2.com/index.pl?node=Simtec%20fault%20code%20listing