I'm having the "service engine soon" light come on. it shows a trouble code that's not in the books. It will come on then go off. mostly in the morning. My car is a 91' Z-28 5.7L. I'm using the paper clip method to obtain the codes.

after showing code 12 it flashes

2 (flashes)..pause..5 (flashes)


the car runs good?????

 

Thats a code 25. This means High temperature at manifold air temperature sensor OR Vacuum switching valve circuit open or shorted to ground OR High voltage at ATS sensor. This is from the FAQ portion of this site. The or's are in there because it depends on your specific year and engine. Consult a service manual to figure out which is the culpret.