Code P0420 Keeps coming up...what is this?
Code P0420 Keeps coming up...what is this? My younger brothers 00 SS Keeps setting this code, we keep clearing it with autotap and it comes right back a few minutes later. Anyone know what this code is? What would need replaced? <img src="graemlins/gr_guns.gif" border="0" alt="[guns]" />
TECH Enthusiast
Joined: Nov 2001
Posts: 652
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From: Newport Coast, Ca
Re: Code P0420 Keeps coming up...what is this? I dont know if this helps but checking the A-tap booklet guide it says:
P0420--- Catlyst System Efficiency Below Threshold
(Bank 1)
Does he have cats on his car? i bet he does. Something is wrong with his catalytic converters maybe????
P0420--- Catlyst System Efficiency Below Threshold
(Bank 1)
Does he have cats on his car? i bet he does. Something is wrong with his catalytic converters maybe????
Re: Code P0420 Keeps coming up...what is this? Hey thanks for the quick reply. Yes he has cats on the vehichle and he has roughly 45K on the vehichle.. You think the cats need replaced?
TECH Resident
Joined: Nov 2001
Posts: 894
Likes: 0
From: Chicago, IL
Re: Code P0420 Keeps coming up...what is this? The cats aren't lighting off and cleaning the exhaust properly. Does he have headers, specifically mid-lenghts or longtubes? Those types of headers move the cats farther away from the engine so they won't heat up as quickly and won't run as efficiently. If that's it, just get some O2 sims.
Mine did this when my stock cats were annihilated from a bad crank position sensor. I got the code every five minutes or so while driving. I replaced the sensor but by that time, the cats were toast. I gutted them and now I get the code once every few weeks at the most.
Mine did this when my stock cats were annihilated from a bad crank position sensor. I got the code every five minutes or so while driving. I replaced the sensor but by that time, the cats were toast. I gutted them and now I get the code once every few weeks at the most.
Re: Code P0420 Keeps coming up...what is this? Yeah same here, not sure why I got the code anyway. My car was stock at the time in fact. *shrug* O2 simms and no codes, and no problems, almost a year later.
Dope
Dope
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TECH Apprentice
Joined: Nov 2001
Posts: 398
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From: Sterling Heights Mi
Re: Code P0420 Keeps coming up...what is this? I've set it a couple times. About 6 times total. Plus I set a 420 and a 430 last night on the way home from work. Didn't drive it today though. I'm just gonna gut them, and get some simms. Anybody know what a 430 is?
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Verdad Gallardo TECH Resident
Joined: Nov 2001
Posts: 894
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From: Chicago, IL
Re: Code P0420 Keeps coming up...what is this? Blackhawk: you still have the stock cats I take it? Mine were definitely killing power but only because they were almost completely blocked (see above post). The back of the brick inside looked like mud, no honeycomb pattern like there should be.
TECH Apprentice
Joined: Nov 2001
Posts: 398
Likes: 0
From: Sterling Heights Mi
Re: Code P0420 Keeps coming up...what is this? Yep stock cats and manifolds with a loudmouth. I'm thinking about putting small bullets in place of the cats. It all depends on how loud it is. If the 2 bullets don't do the trick, then I'll replace the resonator with a bigger bullet. If that's not enough then 2 more bullets in the tail pipes.
So far I haven't set any other codes to date.
So far I haven't set any other codes to date.
TECH Enthusiast
Joined: May 2004
Posts: 709
Likes: 0
From: San Jose, NorCal
I too just recently through this code. Um, I'm not exactly sure but how do 02 simms work. And this replace 02 sensors ? I dont know , sorry. If theres any reason why I shouldnt use 02 simms, then just replace the cat with say what.. an SLP or magna?
BTW, this is for my 3.8 with stock headers.
BTW, this is for my 3.8 with stock headers.
The PCM monitors the catalytic converters by watching the front and rear O2 sensors as they switch from rich to lean. With a properly functioning catalyst the rear O2 will switch slower then the front O2 if watched on a scan tool. If you are setting a P0420 it is because the catalyst has deteriorated to the point that it cannot control emissions as designed and the exhaust gases will flow through it faster then a good cat. You will see the front/rear 02s mirror each other on the scan tool when this happens and it means the cat is junk. I believe that P0420 is for the drivers side cat so it will need replaced. GM warranties cats for 8 years or 80,000 miles whichever comes first. Take it to the dealer, they will replace the faulty converter for free. 
