1998 Chevy Astro scanner code

One theory may be that the maf code was the original problem. Whetever it's source is may have caused a conditon the resulted in a misfire or a rich condition that may have damaged the converter. The po 420 almost always ends up being a convererter failure that may be helping to cause the misfire. Hard to say, but I expect this may be at least 2 sources of trouble. Pull the plug on # one cyl. And get a look at it and see if it sheds some clues, rich or lean, coolant or oil burn etc. This may help to decide which code to first focus on and form a plan on how you're going to start testing.