87\' blazer service engine soon lite/code 12 picked up an 87' blazer with 305 fi over the weekend. seems to run fine, guages read ok, but recently seems to sometimes start hard. going to give it a complete tune up soon. not long after i got it home, the service engine soon lite came on, and it stays on. comes on not long after starting up. took it to autozone and they pluged in the little key thing into computer port and told me code 12 but really couldnt tell me much more. they said it has to be taken to a garage and ran a diagnistic on. anybody else tell me what the problem could be and save me money and time ? the autozone guys said it could be a oxygen sensor or even a loose wire. thanks ahead of time for any and all help.