2003 V6 repeatedly throwing P0420 code

always wanted a pony · November 22nd, 2011, 07:22 PM

I've had my mustang for about 3 years now and over the course of that time this code has popped up a few times. MPG has never gone down and I've never had a performance problem with it. My initial thought was to replace to fuel cap cause I know that can be an issue on this generation. I have aftermarket flowmaster dual exhaust and I've had that on there for almost as long as I've had the car.

The weird thing is, when I turn the check engine light off it will stay off for a few months then come back on.

Any ideas what it is and where I should look for it?

Kyle · November 22nd, 2011, 08:04 PM

Catalytic converter

always wanted a pony · November 23rd, 2011, 11:19 PM

It isn't the cat.

Zatrekaz · November 23rd, 2011, 11:44 PM

If you don't know what the code means, how do you know its not your cats?

Go to an auto store and have them look up the code, problem solved.

xtreme 2 · November 23rd, 2011, 11:52 PM

true, how do you know it's not the cats? Do you not have them anymore? This can cause a problem if you did not shut off the rear o2s.

Kyle · November 24th, 2011, 07:33 AM

EVERY vehicle I have seen with a P0420 is cat... its bad that its a code I don't even have to look up..

But go spend money on whatever you want

SON1C · November 26th, 2011, 09:29 PM

p0420 was a nightmare for me, its more than likely clogged cats

Womkeith · November 28th, 2011, 02:41 PM

Possible exhaust leAk b4 the cats as well

mustangmike! · November 28th, 2011, 02:56 PM

Whats the discription ?

xtreme 2 · November 28th, 2011, 03:40 PM

the code description? "Catalyst System Efficiency Below Threshold (Bank 1)"

It's the rear driver side o2 sensor noticing that the flow is decreased. Either an exhaust leak, a clogged cat, or a bad o2 Sensor.

Kyle · November 28th, 2011, 04:09 PM

CANT YOU PEOPLE READ??

ITS NOT THE CAT...

But since he knows what its NOT, Im wondering why he doesnt know what it is...

SON1C · November 28th, 2011, 04:13 PM

Originally Posted by Kyle

CANT YOU PEOPLE READ??

ITS NOT THE CAT...

But since he knows what its NOT, Im wondering why he doesnt know what it is...

relax. and ill bet its the cat, paypal.

Xzavier247 · November 28th, 2011, 04:43 PM

Originally Posted by SteeD43

relax. and ill bet its the cat, paypal.

mustangmike! · November 28th, 2011, 05:48 PM

Originally Posted by Kyle

CANT YOU PEOPLE READ??

ITS NOT THE CAT...

But since he knows what its NOT, Im wondering why he doesnt know what it is...

Kyle · November 28th, 2011, 06:31 PM

Originally Posted by SteeD43

relax. and ill bet its the cat, paypal.

Look at the very first post AFTER the OP.....

SON1C · November 28th, 2011, 10:47 PM

obviously i read that before all of my posts, i stand by my bet

xtreme 2 · November 29th, 2011, 09:57 AM

Originally Posted by Kyle

Look at the very first post AFTER the OP.....

Do you also realize homeskillet has no posts after him saying it's not the cats. We just want to know HOW he came to the idea that it's not the cats. The code is specifically for a clogged cat, so I don't think it's a dirty air filter...

Kyle · November 29th, 2011, 12:56 PM

Im a service advisor at a dealership, I know what I am talking about 99.9% of the time when it comes to codes

xtreme 2 · November 29th, 2011, 01:01 PM

there is always that .1% when you are wrong...

and I don't see you offering any other suggestions... obviously the guy isn't too worried about it if he isn't responding back... ah well...

Kyle · November 29th, 2011, 01:10 PM

Im not going to offer him any more suggestions.. I told him the problem from the beginning, he said I was wrong, so Ill keep my thoughts to myself..

Im not the one asking for help

November 22nd, 2011, 07:22 PM	#1
always wanted a pony Regular 2003 Mustang Join Date: Oct 2010 Location: Cincinnati, OH Posts: 58 iTrader: 0 reviews	2003 V6 repeatedly throwing P0420 code I've had my mustang for about 3 years now and over the course of that time this code has popped up a few times. MPG has never gone down and I've never had a performance problem with it. My initial thought was to replace to fuel cap cause I know that can be an issue on this generation. I have aftermarket flowmaster dual exhaust and I've had that on there for almost as long as I've had the car. The weird thing is, when I turn the check engine light off it will stay off for a few months then come back on. Any ideas what it is and where I should look for it?

November 22nd, 2011, 08:04 PM	#2
Kyle Regular 03 DSG GT 12.82@111.23mph Join Date: Jan 2007 Location: Baytown, TX Posts: 285 iTrader: 3 reviews	Catalytic converter __________________ 2003 DSG GT AUTOMATIC - 12.82 1/4 @ 111.23 mph

November 23rd, 2011, 11:44 PM	#4
Zatrekaz Wax on, Wax off 2003 Mustang V6 16.8@83.3 Join Date: Aug 2007 Location: Mississippi Posts: 6,954 iTrader: 15 reviews	If you don't know what the code means, how do you know its not your cats? Go to an auto store and have them look up the code, problem solved. __________________ Black Bullitt Rims FT/FS Looking to Sell or Trade for Silver FR500s Looking to buy a lot of 99-04 Interior Pieces If you have any for sale, PM me

November 23rd, 2011, 11:52 PM	#5
xtreme 2 Hardcore Enthusiast 2001 v6 premium Join Date: Nov 2008 Location: Tulsa, OK Posts: 1,340 Blog Entries: 1 iTrader: 0 reviews	true, how do you know it's not the cats? Do you not have them anymore? This can cause a problem if you did not shut off the rear o2s. __________________ Screw it, coming to play with the big boys... 5.4L Vortech COMING SOON!

November 24th, 2011, 07:33 AM	#6
Kyle Regular 03 DSG GT 12.82@111.23mph Join Date: Jan 2007 Location: Baytown, TX Posts: 285 iTrader: 3 reviews	EVERY vehicle I have seen with a P0420 is cat... its bad that its a code I don't even have to look up.. But go spend money on whatever you want __________________ 2003 DSG GT AUTOMATIC - 12.82 1/4 @ 111.23 mph

November 23rd, 2011, 11:19 PM	#3
always wanted a pony Regular 2003 Mustang Join Date: Oct 2010 Location: Cincinnati, OH Posts: 58 iTrader: 0 reviews	It isn't the cat.

November 26th, 2011, 09:29 PM	#7
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	p0420 was a nightmare for me, its more than likely clogged cats __________________ 238rwhp/263rwtq Drew's Build Thread

November 28th, 2011, 02:41 PM	#8
Womkeith Regular 2002 Mustang All day Join Date: Oct 2010 Location: Corpus Christi Posts: 158 iTrader: 0 reviews	Possible exhaust leAk b4 the cats as well __________________ Mavs 4 Blazers 2 Mavs 4 Lakers 0 SWEEP!!! Mavs 4 Thunder 1 <---Going to FINALS!!! Mavs 4 Heat 2 <------2011 NBA CHAMPS!!!

November 28th, 2011, 02:56 PM	#9
mustangmike! I Post Entirely Way Too Much 2004,1994,1967 14.37 @ 96 Join Date: Aug 2008 Location: Texas Posts: 6,018 iTrader: 1 reviews	Whats the discription ? __________________ 227whp N/A V6 Now w/ 8th Mile 8s ! How To Jmod Your Late Model 4R70W

November 28th, 2011, 03:40 PM	#10
xtreme 2 Hardcore Enthusiast 2001 v6 premium Join Date: Nov 2008 Location: Tulsa, OK Posts: 1,340 Blog Entries: 1 iTrader: 0 reviews	the code description? "Catalyst System Efficiency Below Threshold (Bank 1)" It's the rear driver side o2 sensor noticing that the flow is decreased. Either an exhaust leak, a clogged cat, or a bad o2 Sensor. __________________ Screw it, coming to play with the big boys... 5.4L Vortech COMING SOON!

November 28th, 2011, 04:09 PM	#11
Kyle Regular 03 DSG GT 12.82@111.23mph Join Date: Jan 2007 Location: Baytown, TX Posts: 285 iTrader: 3 reviews	CANT YOU PEOPLE READ?? ITS NOT THE CAT... But since he knows what its NOT, Im wondering why he doesnt know what it is... __________________ 2003 DSG GT AUTOMATIC - 12.82 1/4 @ 111.23 mph

November 28th, 2011, 10:47 PM	#16
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	obviously i read that before all of my posts, i stand by my bet __________________ 238rwhp/263rwtq Drew's Build Thread

November 29th, 2011, 12:56 PM	#18
Kyle Regular 03 DSG GT 12.82@111.23mph Join Date: Jan 2007 Location: Baytown, TX Posts: 285 iTrader: 3 reviews	Im a service advisor at a dealership, I know what I am talking about 99.9% of the time when it comes to codes __________________ 2003 DSG GT AUTOMATIC - 12.82 1/4 @ 111.23 mph

November 29th, 2011, 01:01 PM	#19
xtreme 2 Hardcore Enthusiast 2001 v6 premium Join Date: Nov 2008 Location: Tulsa, OK Posts: 1,340 Blog Entries: 1 iTrader: 0 reviews	there is always that .1% when you are wrong... and I don't see you offering any other suggestions... obviously the guy isn't too worried about it if he isn't responding back... ah well... __________________ Screw it, coming to play with the big boys... 5.4L Vortech COMING SOON!

November 29th, 2011, 01:10 PM	#20
Kyle Regular 03 DSG GT 12.82@111.23mph Join Date: Jan 2007 Location: Baytown, TX Posts: 285 iTrader: 3 reviews	Im not going to offer him any more suggestions.. I told him the problem from the beginning, he said I was wrong, so Ill keep my thoughts to myself.. Im not the one asking for help __________________ 2003 DSG GT AUTOMATIC - 12.82 1/4 @ 111.23 mph

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