Posted this on LS1tech too, but:

Both my front O2 sensors are throwing codes. SES light just came on and had OBDII scan tool read 2 bad O2 sensors. I replaced one of them 3000 miles ago and the other maybe 1500 miles ago? Car was tuned w/flowmaster muffler not to long ago and ran good. There was an occasional misfire the last few weeks, no codes. I started to replace plugs. Found a cracked one and replaced it. Ran better for a day. Also- I did some recent mods to my flowmaster, 1 inlet to 2 outlets(2.5"). Had a muffler shop weld one of the outlets so I had one tail pipe coming out. Just trying to get it quieter and look like a V6. I thought(NOT a good thing) that one 2.5" inlet and one 2.5" outlet would be fine, just loose a few hp (but apparently this may be bad). Did I screw up the exhaust gas flow and back it up to my O2s? I'd like to keep the exhaust the way it is for now unless it is the problem. I should just get a better muffler but I wanted to mod what I had out of curiosity. Now the misfire I had happend before exhaust mod. I guess I am going to unplug the exhaust and see what happens. Any thoughts?