Crank but won't start.

About 2 months ago I replaced the head gasket. When I tried to crank it, it didn't start the first try like it used to. The second or third try would get it started. If I gave it gas after starting, it would try to stall out until it was warmed up then the car ran fine. Other than poor fuel consumption. NOW it all of a sudden won't start. When it does, it only run a few seconds before stalling out. And I can smell gas. I opened the breather cap to find the screen to my MAF sensor laying on the filter. From past experiences every time this screen is moved, it effects the way the car runs. For the most part it would cut the engine if this screen is out of leaning back touching the housing of the sensor. I put the screen back in but nothing changed. When I did a diagnostic test it reads code 13 which is the HO2 sensor. I haven't replaced it yet because I'm hearing that won't cause it to do those things alone. I powered the MAF and took readings when blowing thru it and it read over 2.5v. The repair manual says it should be between.5-1.5v Could it be that I'm blowing a little to hard?
I had a head full of hair. Now it's all pulled out. Can someone please help?