Help! 2000 GMC Yukon SLT, service engine light

[Slam Dunked]

Hey everyone,

Sorry I haven't made an intro thread or introduced myself properly. I live in Washington state and I currently own (among my other vehicles) a 2000 GMC Yukon SLT. Lately it's been giving me some problems and I'm out of answers and low on cash so any help you guys could give me would be great.

Here's what I've replaced in the past couple months:

Plugs
Plugwires
Fuel Filter
Air Filter
Injector Cleaner
Cleaned MAF and throttle body

I've already had the fuel pump replaced (found that one out the hard way). So here's my dilemma. Truck was running like ****. I didn't know what the deal was. I took it to my local mechanic (I always take my cars to him if it's beyond my expertise) and he told me the gap on the plugs had closed. Didn't know how this was possible because I had JUST replaced the plugs the night before. I'm no idiot, I've been working on cars for awhile. I never dropped the plugs, never banged them up, nada. He told me the gap was closed and they weren't creating spark. He knows I'm not an idiot and we're both kinda stumped. Possibly the piston is hitting the plug?? I dunno, anywho, he put in fresh plugs and the truck was running great.

So now, the service engine light is back on. I don't know what's going on. Truck is running fine for the most part other than I'm getting HORRIBLE gas mileage. I'm at a little over 1/4 tank and I only have 175mi on this tank so far. WTF? Can anybody help me out? Thanks in advance!!!

 

[NJTahoe]

First thing is to get the codes pulled. If you haven't done that yet, that is your best bet to figure out what is going on.
As far as the spark plugs closing the gap, well, Are you sure you picked up the right ones, or better yet, maybe the guy behind the counter just grabbed the wrong ones for your Kon.
Main priority,
Go to your local parts store and get the codes pulled. Find out whats failing and why.
Good Luck!