My 1994 bonneville stalls after the engine warms up. It seems to happen whenever you take your foot off the gas. If you give it a little gas while braking, it keeps running. The service engine light stays on. When I took it in to have the code read I was told the connection under my dash was not working and they couldn't read it. The oil pressure and water temp is within normal ranges. What should I be looking for?

Optional Information:
1994 Pontiac bonneville 6 cyl.

Already Tried:
spark plugs & wires and air filter.