Running on 3 out 4 cylinders

What is your question?

Engine light came on, then I had a sudden loss of power. My car was burning oil for a bit before this happened. I hooked it up to the machine at auto zone, and my husband either checked, switched, or replaced everything that the machine said was wrong..including the mass air something or other...but nothing has changed. This happened at 93,700..now I'm at 98,300, and still can't figure out what is wrong. My husband is guessing that it could be valves? If that's the case..is it a diy job?