91 PA check engine but no codes

I get the check engine light on when im driving on the highway, usually over 70 km/h and I keep the code reader in the car with me... Ive tried it many times just pulling the car over and turing off and placing code reader in but it will only flash 12.... is there a possibility that the diagnostics computer has kicked the bucket?

Hmm, if a problem was detected long enough to trip the light and your tyring to access it right away and nothing shows, that's really odd. Even after the problem is corrected, the ECM normally waits so many cycle counts to determine the problam area cured.

I would have to say the recourse is to have a scanner with live data stream capapbility hooked up and create the problem, then have the scanner do a snap shot of the ECM status at that time. Which 9 of 10 times means, gotta go to the pros, sorry.