egr valve
 

i have code 404 coming up and comes right back when i clear it , i'm getting crappy gas mileage could this be part of the problem , somebody told me it could cause bad gas mileage

You don't say what year, model, and engine size but, in general a malfunctioning EGR valve can indeed cause spark knock and resulting timing retardation and fuel dumping.

sorry its a 2001 gmc 1500 with a 4.8

The EGR valve is a solenoid, pintle valve, and pintle position sensor all in one. Try cleaning the valve portion (search for how-to). If that doesn't work, get it checked out at a shop. I think an EGR valve is a little too expensive just to throw a new one on.