Engine light came on and thenn went off and is now back on...?

we have a 2003 mazda mpv. the engine came on a few months back. we had to take our car in for some other wok on it (nothing to do with the light). when we got the car back the light was off. a few days ago it came back on and is now staying on. at the same time that it came back on the car started to make a shivering and sputtering sound that it did not make before. any one have any idea what is going on? it started when my husband bought a little gas at a different station than he usually buys it at. we have tried several different gas additives but none seem to work. we have to have it inspected in April but not sure if it will p like this. PLEASE HELP.