1995 Monte Carlo runs rough

I just bought a 1995 Monte Carlo with a 3100 V6 120K miles. The car ran & ideled fine when purchased with no problems. I drove the car to work today, no problems & as soon as I started to come home today the car started idling rough, accelerating sluggish with a noticable "hollow popping" sound from the engine when accelerating. The check engine light came on and stayed on. I am not able to check the flash codes because I have a 12 pin plug with only two wires at A & M. Is there a way to "jumper" this connector to check for flash codes. The car's previous owner didn't know much about the car because he had just purchased it, but he had a new battery, radiator, thermostat and hoses put in as well as a new serp belt. The cooling fan was loose, which I tightened. I changed the oil and installed a new tesioner pulley. I didn't pay much for the car and can't afford to put much more into it. Please help.