What did I do?
What did I do? 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? Please don't tell me it was stupid, I'm aware of that 
. 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? Please don't tell me it was stupid, I'm aware of that Last edited by TedsB4Csled; Jun 21, 2008 at 02:25 PM. Reason: sp
I guess I'll talk to myself Anyway, checked the MAF, it is an SLP(another bad mistake). It's clean, but I did disconnect it and ran the car down the road. Ran better than before! I think I might throw stock MAF back on. Codes were P0174 and P0171. Also gonna swap O2 sensors and see what happens(front to rear) before switching MAF. Made sure MAF and wiring did NOT strike or rub under hood. There was one other possibility, but I'll have to decipher codes again. Fun, fun!
Anyway, checked the MAF, it is an SLP(another bad mistake). It's clean, but I did disconnect it and ran the car down the road. Ran better than before! I think I might throw stock MAF back on. Codes were P0174 and P0171. Also gonna swap O2 sensors and see what happens(front to rear) before switching MAF. Made sure MAF and wiring did NOT strike or rub under hood. There was one other possibility, but I'll have to decipher codes again. Fun, fun! 
TECH Senior Member
Joined: Dec 2001
Posts: 5,399
Likes: 3
From: Jacksonville, FL
P0171 and P0174 at the same time is probably not the 02's. Most likely cause is a vacuum leak or unmetered air - and yes the PCV being removed or incorrect can cause it also. Visit gearchatter.com for your year and click on the description/diagnostics for the codes.
Thanks man, that's probably what it is. The PCV grommet was tore up so I took it and the PCV off and just connected the hose to the TB. Got the DTC description too. Didn't think the 2 would go bad like that, and I panicked, thought the exhaust screwed it up. Doing front springs right now, I'll get a new grommet tomorrow.
