2001 5.3 Silverado running rough.....SES light on

Ok so the other day my SES light came on and it started running a little rough. I already had plugs and wires on hand so I put them it all was well....so I thought. The next day it started all over again! Sometimes the SES light will even flash and it smells a little gassy "fuel" under the hood.

I'm sure this could be about 50 different things but what could be the likely causes here?

Start with the code and post back results.

Well the problem turned out to be a leaky intake gasket. Now the same day I got it back the SES light comes back on but in seems to be running ok. Codes are P0332 and P1133. It seems my gas mileage could've went down a bit also.