p0420 code

G_T_O

TCG Elite Member
May 10, 2009
2,301
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Yesterday I was driving and noticed the Check Engine light went one, so I ran over to autozone and the code is p0420. I have no clue what the help could be wrong. Says the cat could be bad but it doesnt smell like eggs or anything. Had one of my buddies look at it and he says the cat is still working. Also it could be a large vaccum leak. But my vac seems fine because the boost gauge says it is sucking 21 pounds of vac. I am though hearing a hissing noise coming from somewhere around the throttle body. I recently put a throttle body spacer in but i made sure it was tightened all the way down. I check twice on that but I am still hearing the hissing noise and we have no clue what the hell could be hissing any ideas anyone?
 

G_T_O

TCG Elite Member
May 10, 2009
2,301
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dont know where it was coming from. Sprayed everything around there with water did it for 15 minutes and we couldnt see anything or find anything. The vaccum is still at what it should be but we just can seem to pinpoint it. If the Check Engine light goes back on which it hasn't for two days im going to take that gasket off and throw it away.
 

Alacran

GM fanatic
Jan 18, 2009
1,152
23
Chicago
^^^water is not going to do anything! Its not like you are looking for a tire leaking air. you need something that is flammable (carb or tb cleaner) so the engine speeds up to find the vacuum leak. However, with the blower its very hard to do that and not hurt the coating on the rotors.

Also, you are measuring vacuum after the leak or downstream from the leak so you cant find it that way. If your gauge was before the leak then it would be obvious using the gauge.

IMO you should replace the gasket with a stock to see if the hissing goes away. if it does than you need to use RTV to seal the reusable one.
 

M@

Somebody buy this thing
Nov 26, 2007
2,871
2
P0420 would have nothing to do with a vac leak.

I'm not saying there's not one present...TB spacers aren't really good for anything BUT causing vacuum and coolant leaks, but if there is a leak, it has nothing to do with the SES light.

If you have a high-flow cat, the code is pretty normal. You'll need to delete it with a tuner, get an O2 simulator, or just live with the light. If it's a stock cat, your O2 sensor is bad, or the cat is not working properly. If there's no cat...well, I don't think I need to explain that one.

Definitely check for a vac leak though, that's not helping anything, but pulling 21 inches of vacuum sounds to me like there's no leaks. The hissing might well be the IAC valve functioning normally.
 

G_T_O

TCG Elite Member
May 10, 2009
2,301
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Yes it has the stock cat on there and I have brand new o2 sensors in it. I deleted the code and it has not come on for the past 3 days or so. Im pretty shure there is nothing wrong maybe that IAC valve functioning normally like you said. Just never heard it before so I was kinda wondering what the hell it could be. Thanks guys for the help :).
 

M@

Somebody buy this thing
Nov 26, 2007
2,871
2
Yes it has the stock cat on there and I have brand new o2 sensors in it. I deleted the code and it has not come on for the past 3 days or so. Im pretty shure there is nothing wrong maybe that IAC valve functioning normally like you said. Just never heard it before so I was kinda wondering what the hell it could be. Thanks guys for the help :).

Have you recently changed intake setups?

A more free flowing intake will make the noise from the IAC more pronounced, and it does sound like a hissing. If the intake was changed recently, this might just be the first time you're noticing it.

What brand of O2 sensor was used when you replaced them? If it was a Bosch sensor, it might well be dead, they're notoriously terrible on these motors. Also, if the u-bend was replaced, and the sensor was relocated further from the cat when it was done, that can also trigger the P0420 code.
 

G_T_O

TCG Elite Member
May 10, 2009
2,301
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I just put in a K&N Drop in Air filter because I am too poor to buy an intake right now :( ! Yes I have two Bosch o2 sensors in it them seem to be doing fine right now. They are working I checked them the other day. Everything seems to be fine but my gas mileage it is like 15.1 but I have the Autolite 104s in them so Im pretty sure that is why it is like that otherwise it runs fine.
 
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