How do I check ecc code

I have a 1992 Pontiac Bonneville ssei. I have ecc. My temp is blinking. My heater core had went and i had a friend replace it he yanked wires down and i made him stop i completed the job myself. My compreser doesn't work anymore my temp light blinkd and my econ doesn't come on. How do I check the codes and what do you think is wrong.