04 C5 throwing p0300 after header install help needed!!!
Thread Starter
Instructor
Joined: Mar 2014
Posts: 149
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From: Alpharetta Georgia
04 C5 throwing p0300 after header install help needed!!!
I recently put on XS Power Headers with catless exhaust and now my car is throwing the code P0300 which is multiple cylinder misfire. During the install I noticed the bolts going into the head for the manifolds where missing on both sides closest to the firewall but the one on the drivers side was actually broke flush with the head, so I didn't attempt to fix it and bolted up the headers anyways thinking that it wouldn't leak because it wasn't before but now it is. I plan on fixing that this weekend, anyways the cars been driving fine up until this morning I started it up to go to work and it started shaking really bad and the check engine light started flashing so I shut it off. I noticed an O2 sensor wire going to the drivers side header looked like it had been melted a little bit to the header and I'm wondering if that is the possible cause? I took off the drivers side valve cover and checked all the springs and push rods and everything looked great so I'm distraught all help would be appreciated!!
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That melted O2 wire can definitely be the cause. Melted O2 wire means improper feedback signal from O2, computer is either doing to dump or pull all the fuel it can depending on which way the O2 signal went. Definitely fix that.
Also address the broken bolt. It may not leak now, but it WILL end up leaking. Once it starts leaking, it'll start throwing your O2 readings on that side off and you'll end up fighting a no win battle unless you fix that broken bolt.
Also address the broken bolt. It may not leak now, but it WILL end up leaking. Once it starts leaking, it'll start throwing your O2 readings on that side off and you'll end up fighting a no win battle unless you fix that broken bolt.
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some other things to look for would be broken spark plugs... they are very easy to break when installing headers if you don't remove them prior or replace them afterwards... and check to make sure your wires are fully engaged on the spark plugs and coil ends... your issue should be pretty obvious once you find it so I wouldn't worry too much
Le Mans Master
Joined: Aug 2005
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St. Jude Donor '13-'14-'15
Yes pull out all the plugs again and re-check the gaps--as sometimes on installation one will bump something and close up
ALSO check the wires to make sure none of them are touching the headers or almost touching
Make sure when plugging the wires onto the coils you hear/feel a definite "click "
and same for the plug end but the click is more subtle and more of a feel
Fix the 02 pigtails----A broken melted 02 wire would affect ALL the plugs on the side of the engine that has the melt--and not cause a misfire code but rather an 02 code
Also P0300 should be a "random misfire" and not a multiple misfire
ALSO check the wires to make sure none of them are touching the headers or almost touching
Make sure when plugging the wires onto the coils you hear/feel a definite "click "
and same for the plug end but the click is more subtle and more of a feel
Fix the 02 pigtails----A broken melted 02 wire would affect ALL the plugs on the side of the engine that has the melt--and not cause a misfire code but rather an 02 code
Also P0300 should be a "random misfire" and not a multiple misfire
Thread Starter
Instructor
Joined: Mar 2014
Posts: 149
Likes: 9
From: Alpharetta Georgia
it was definitely the O2 sensor because I unplugged the melted side and the car ran like a champ again, although it reeks of fuel lol. I purchased pig tails from ACDelco and will be installing them on the factory O2 thanks for the help!
