Check Engine light

[Tate]

Hey Charlie,
I got the damn check engine light on again, stopped by autozone and the codes are P0420 and P0430. Their telling me it's the front O2 sensors throwing these not the rear. I got O2 codes from the rears after the headers but used the spark plug antifowlers and that cleared up the codes but why would I get codes for the front O2's. I got a check engine light last week as well and talked to Onstar and they told me it was something to do with the emissions, but said they couldn't give me the codes (legal reasons). I pulled the neg. on the battery and cleared them out and here it is a week later and I have them again. I also noticed that I can't get anything better then about 12 MPG and I'm thinking that has to do with the front O2's as well.
What are the codes and any idea what I can do to ensure they don't come back, I already cleared them by pulling the neg. on the battery for a bit.
TIA,
Tate

[GRRRR8]

That is for the rears! Cat efficiency codes are what those codes are. You will need to turn them off with a tune.

[Andy@Livernois]

yep 0420 0430 are for rears...

[Tate]

Alright cool, he said it was the fronts. Why is it with the spark plug antifowlers I will go for a week or so without throwing a code then out of the blue it does it?
Tate

[GRRRR8]

Right now I am on day 8 with no foulers. It depends on a few factors, but that is what the issue is for sure.

[todds87ss]

Charlie,
While I agree that those are cat codes, the rears should not be affecting his gas mileage. Something else is going on causing the Cat codes.

[Andy@Livernois]

the rears always effect gas mileage from my experience... if the code's on it goes into thermal protect mode from my understanding. This adds a bunch of fuel to compensate and cool the converter...

[todds87ss]

When I remove them for a header install, the car always runs fine, just throws the codes. The fronts control fuel trims.

[GRRRR8]

You removed/turned them off, I dont believe he has had it tuned yet after the LT install.

[todds87ss]

car just throws codes until I get them tuned out. Does not run poorly. Anyone else confirm, or am I out to lunch...again?

[GRRRR8]

He didnt say ran poorly, just fuel economy. Depending on where he s located and temp this would be normal without a tune for the combo.

[-Ray-]

It has not affected my mileage on the Interstate. In town is different. I'm getting around 12 mpg in town now.

[Andy@Livernois]

Like I said sounds pretty normal to me. Until it is tuned mileage should suffer as it thinks the cats have failed...

[Tate]

O.K. I'm waiting on the tune right now, anyone want to get VMS to make another cable and send it out that would be great. So yes the codes will be killed then. As far as the car running, it's great just throws a damn CEL every week or so and in town fuel mileage has went to crap (11-12MPG). On the road I can pull around 27.5-28.5 if I keep it between 62-68 just in the city it's hurting. Thanks for the help, just gotta wait for my tune to get here.
Tate

[todds87ss]

He didnt say ran poorly, just fuel economy. Depending on where he s located and temp this would be normal without a tune for the combo.

Ahh, yes. Bad economy...headers...heavy foot...no tune! The light just came on.

[-Ray-]

O.K. I'm waiting on the tune right now, anyone want to get VMS to make another cable and send it out that would be great. So yes the codes will be killed then. As far as the car running, it's great just throws a damn CEL every week or so and in town fuel mileage has went to crap (11-12MPG). On the road I can pull around 27.5-28.5 if I keep it between 62-68 just in the city it's hurting. Thanks for the help, just gotta wait for my tune to get here.
Tate


I'm in the same boat as you. My CEL is on all the time though.