OBDII Code P0420

ASM92 · October 31st, 2011, 04:59 PM

A few months ago the service engine light came on so I scaned for code and it was the Catalyst P0420 code. At the time I had a pretty big exhaust leak, but I fixed it and cleared the code out, and the light has not come on since then.

Just a few days ago after a few thousand miles thes service engine code came back on, so I scaned it the same code came up.

I have checked for vacuum leaks, any misfires, and it doesnt smoke at all so there is not oil leaking into the exhaust valve. If anyone has an idea of what could be wrong please let me know.

Is there a way to tell if a catalytic converter is bad without taking it off?

The car has 148,900 miles on it.

walt1120 · October 31st, 2011, 08:36 PM

Usually this code is thrown when someone deletes there catalytic converter.

You may have a bad cat but it could just be an o2 sensor.

-First i would check for any exhaust leak the best you can
-Then you should replace your downstream o2 sensors. These are the ones that are in or close to the cat converter.
-If that still doesnt work then ive heard you can put a gallon of lacquer thinner in the gas take when its under half full. This will help clean out the cat but i would do it at your own risk.
-If all else fails then just buy a magnaflow cat off ebay for like $60 and replace the cat

ASM92 · October 31st, 2011, 10:08 PM

Thanks for the info I'll definately check the rear O2 sensor. If it's just that then I'll replace it.
If it turns out to be the cat then I will for sure get some magnaflow catalytic converters and do away with the secondary cats, and get more sound out of it.

I have also checked four or more times for any exhaust leaks, and even tightened all the manifold bolts just to be sure.

If I replace the cats should I get some with the rear O2 sensor holes in them or should I just get some MIL eliminators and not use the rear O2's?

Im not to sure about the lacquer in the gas tank. I wont try it on the stang but I got an old Dodge Ram Charger sitting around so maybe I will try it with that and see if it gets hurt in any way doing that.

Skargon · October 31st, 2011, 11:10 PM

Originally Posted by ASM92

Thanks for the info I'll definately check the rear O2 sensor. If it's just that then I'll replace it.
If it turns out to be the cat then I will for sure get some magnaflow catalytic converters and do away with the secondary cats, and get more sound out of it.

I have also checked four or more times for any exhaust leaks, and even tightened all the manifold bolts just to be sure.

If I replace the cats should I get some with the rear O2 sensor holes in them or should I just get some MIL eliminators and not use the rear O2's?

Im not to sure about the lacquer in the gas tank. I wont try it on the stang but I got an old Dodge Ram Charger sitting around so maybe I will try it with that and see if it gets hurt in any way doing that.

Try not to over-torque things and make them a pain to fix/replace later.

6ER THEN I 8ER! · October 31st, 2011, 11:28 PM

Delete your cats altogether....if your in an emissions area have the heat shields welded up and use mil eliminators from American muscle to kill the o2 code. Or buy new cats and possibly new downstreams and hope it kills the code.

ASM92 · November 1st, 2011, 09:21 AM

@ 6ER THEN I 8ER: I thought about just getting an o/r H-pipe but I don't want it to be to I guess ricy sounding. On youtube I have heard the H-pipe setup and it doesn't sound to bad but you can't be to sure if that is what it would actually sound like or if its just the camera or video editing or whatever.

I rekilled a deer this early this morning and now have a bad exhaust leak again, so now I need to get under there again and check it out. I hope nothing broke and that it's just a matter of reseting the exhaust pipes and tightening them back up.

I hope it's not to expisive to fix what's causing it to throw a code.

mcqueen · November 1st, 2011, 10:25 AM

I'm going to agree with 6er. Supposing it isn't another exhaust leak, delete your rear cats and run MIL eliminators or if you have a tuner then just shut the rear O2s off.

6ER THEN I 8ER! · November 1st, 2011, 11:11 AM

Ricey sounding with an o/r h pipe? I don't think you'd find it rcey at all with a nice muffler combo

mcqueen · November 1st, 2011, 11:37 AM

Originally Posted by 6ER THEN I 8ER

Ricey sounding with an o/r h pipe? I don't think you'd find it rcey at all with a nice muffler combo

Flow 40's and an o/r H sounds like sex to me

ASM92 · November 1st, 2011, 07:04 PM

I already have true duals with flowmasters and all the stock cats. To keep it legal I was thinking about just cutting of the second set of cats, but I don't have a tuner to shut off the rear O2's. I would have to get some mil's, but until then would it hurt the motor if I just left them unplugged?

mcqueen · November 1st, 2011, 07:17 PM

It would constantly throw codes for the lack of O2 sensors. If you have inspections, you'll need MIL eliminators. And if you ever want to sell the car, it looks shitty when someone comes to look at it and the CEL is on.

6ER THEN I 8ER! · November 1st, 2011, 10:40 PM

Originally Posted by ASM92

I already have true duals with flowmasters and all the stock cats. To keep it legal I was thinking about just cutting of the second set of cats, but I don't have a tuner to shut off the rear O2's. I would have to get some mil's, but until then would it hurt the motor if I just left them unplugged?

Lol it wouldn't be legal with the two rears off...

SON1C · November 2nd, 2011, 04:45 PM

dont buy new 02s for the rear... all those do is monitor if the cats are tehre, if you believe they're faulty swap with the upstream, but honestly i had the same code and i'd bet your cats are clogged, just delete them you'll love the improvement

SON1C · November 2nd, 2011, 04:46 PM

mac o/r h pipe(for our car) from StangMods Mustang Parts & Performance is 180 shipped i think, look around for a discount and you'll get a good deal on a good product.

walt1120 · November 2nd, 2011, 10:54 PM

Originally Posted by SteeD43

dont buy new 02s for the rear... all those do is monitor if the cats are tehre, if you believe they're faulty swap with the upstream, but honestly i had the same code and i'd bet your cats are clogged, just delete them you'll love the improvement

P0420 doesnt have anything to do with upstreams. The code is caused by the downstream o2 sensor saying that the catalytic converter not performing up to par.

So its usually a clogged cat or bad (downstream) o2 sensor

A bad upstream would be throwing rich and lean codes if it was messed up

ASM92 · November 3rd, 2011, 12:28 AM

Originally Posted by SteeD43

mac o/r h pipe(for our car) from StangMods Mustang Parts & Performance is 180 shipped i think, look around for a discount and you'll get a good deal on a good product.

I have been thinking about just doing an h-pipe. I'll be sure to check the mac h out, but I have been looking at one on Jegs.com for 130.

October 31st, 2011, 04:59 PM	#1
ASM92 Newbie 2001 Mustang Have not run official times Join Date: Oct 2011 Location: Madisonville, Texas Posts: 21 iTrader: 0 reviews	OBDII Code P0420 A few months ago the service engine light came on so I scaned for code and it was the Catalyst P0420 code. At the time I had a pretty big exhaust leak, but I fixed it and cleared the code out, and the light has not come on since then. Just a few days ago after a few thousand miles thes service engine code came back on, so I scaned it the same code came up. I have checked for vacuum leaks, any misfires, and it doesnt smoke at all so there is not oil leaking into the exhaust valve. If anyone has an idea of what could be wrong please let me know. Is there a way to tell if a catalytic converter is bad without taking it off? The car has 148,900 miles on it.

October 31st, 2011, 08:36 PM	#2
walt1120 Hardcore Enthusiast 04 Stang 1.0Gallon V6 14.45 @95.8 Join Date: Mar 2010 Location: South Jersey Posts: 2,130 iTrader: 2 reviews	Usually this code is thrown when someone deletes there catalytic converter. You may have a bad cat but it could just be an o2 sensor. -First i would check for any exhaust leak the best you can -Then you should replace your downstream o2 sensors. These are the ones that are in or close to the cat converter. -If that still doesnt work then ive heard you can put a gallon of lacquer thinner in the gas take when its under half full. This will help clean out the cat but i would do it at your own risk. -If all else fails then just buy a magnaflow cat off ebay for like $60 and replace the cat __________________ 2004 Bolt on & Cammed 3.9L V6

November 2nd, 2011, 04:45 PM	#13
SON1C Run ur car, not ur mouth. 2003 Sonic Blue 4.6L Join Date: Jul 2010 Location: Northern Virginia Posts: 4,057 Blog Entries: 1 iTrader: 3 reviews	dont buy new 02s for the rear... all those do is monitor if the cats are tehre, if you believe they're faulty swap with the upstream, but honestly i had the same code and i'd bet your cats are clogged, just delete them you'll love the improvement __________________ 238rwhp/263rwtq Drew's Build Thread

November 2nd, 2011, 04:46 PM	#14
SON1C Run ur car, not ur mouth. 2003 Sonic Blue 4.6L Join Date: Jul 2010 Location: Northern Virginia Posts: 4,057 Blog Entries: 1 iTrader: 3 reviews	mac o/r h pipe(for our car) from StangMods Mustang Parts & Performance is 180 shipped i think, look around for a discount and you'll get a good deal on a good product. __________________ 238rwhp/263rwtq Drew's Build Thread

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October 31st, 2011, 10:08 PM	#3
ASM92 Newbie 2001 Mustang Have not run official times Join Date: Oct 2011 Location: Madisonville, Texas Posts: 21 iTrader: 0 reviews	Thanks for the info I'll definately check the rear O2 sensor. If it's just that then I'll replace it. If it turns out to be the cat then I will for sure get some magnaflow catalytic converters and do away with the secondary cats, and get more sound out of it. I have also checked four or more times for any exhaust leaks, and even tightened all the manifold bolts just to be sure. If I replace the cats should I get some with the rear O2 sensor holes in them or should I just get some MIL eliminators and not use the rear O2's? Im not to sure about the lacquer in the gas tank. I wont try it on the stang but I got an old Dodge Ram Charger sitting around so maybe I will try it with that and see if it gets hurt in any way doing that.

October 31st, 2011, 11:28 PM	#5
6ER THEN I 8ER! Banned Join Date: Apr 2007 Location: Portland, TN Posts: 17,515 iTrader: 3 reviews	Delete your cats altogether....if your in an emissions area have the heat shields welded up and use mil eliminators from American muscle to kill the o2 code. Or buy new cats and possibly new downstreams and hope it kills the code.

November 1st, 2011, 09:21 AM	#6
ASM92 Newbie 2001 Mustang Have not run official times Join Date: Oct 2011 Location: Madisonville, Texas Posts: 21 iTrader: 0 reviews	@ 6ER THEN I 8ER: I thought about just getting an o/r H-pipe but I don't want it to be to I guess ricy sounding. On youtube I have heard the H-pipe setup and it doesn't sound to bad but you can't be to sure if that is what it would actually sound like or if its just the camera or video editing or whatever. I rekilled a deer this early this morning and now have a bad exhaust leak again, so now I need to get under there again and check it out. I hope nothing broke and that it's just a matter of reseting the exhaust pipes and tightening them back up. I hope it's not to expisive to fix what's causing it to throw a code.

November 1st, 2011, 11:11 AM	#8
6ER THEN I 8ER! Banned Join Date: Apr 2007 Location: Portland, TN Posts: 17,515 iTrader: 3 reviews	Ricey sounding with an o/r h pipe? I don't think you'd find it rcey at all with a nice muffler combo

November 1st, 2011, 07:04 PM	#10
ASM92 Newbie 2001 Mustang Have not run official times Join Date: Oct 2011 Location: Madisonville, Texas Posts: 21 iTrader: 0 reviews	I already have true duals with flowmasters and all the stock cats. To keep it legal I was thinking about just cutting of the second set of cats, but I don't have a tuner to shut off the rear O2's. I would have to get some mil's, but until then would it hurt the motor if I just left them unplugged?

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