Thread: 2004 GTP with P0420 diag code

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  1. #1 2004 GTP with P0420 diag code 
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    When I scan it it reads: P0420 Catalyst System Efficiency Below threshold [ Bank1 ] .
    Any one have this code before ? What does it indicate and what would be the fix ?
    I cleared the code. Will see if it comes back.
    Any harm in driving with it ? Or is it just one of the nit picky government mandated
    emissions codes that can be ignored ?
    Thanks !!!
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  2. #2 Re: 2004 GTP with P0420 diag code 
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    Thanks. Car still runs fine. I was just wondering if it could actually be a bad rear O2 sensor, but I know there is probably another code for that.
    I need to take it on a trip in a few days, about 800 miles RT. I don't feel any symptoms of poor performance. Smog not due for a while .....
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  3. #3 Re: 2004 GTP with P0420 diag code 
    SE Level Member 01Blackgtp's Avatar
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    It also depends on if the car is stock or not. Even just cat back exhaust can throw that code on occasion. That code just means it doesn't see enough of a difference between the front and rear 02 sensor. if the car is full stock exhaust I would probably replace the rear 02 before I put a cat in it, or even put a rear 02 extension in. If it still throws the code after that, then sure, put in a good cat. All of the "high flow" and performance cats will give that code anyway, and then you will need the 02 extension.

    My 01 threw that code with every exhaust set up it ran, 2 different cats, rear o2 spacer, then the rear o2 elbow. With the cutout open after a "high-flow"cat it wouldn't do it for weeks, then I would close the cutout and it would come back in 10 minutes. Granted the 01 is far from stock.

    My 04 completely stock cleared that code with the rear o2 spacer and a tank of 109 and nothing else.

    o2 extension is like 10 bucks, rear o2 is like 60, high flow cat is around 100, stock cat is like 500. All of which may correct the code.
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  4. #4 Re: 2004 GTP with P0420 diag code 
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    Quote Originally Posted by 01Blackgtp View Post
    It also depends on if the car is stock or not. Even just cat back exhaust can throw that code on occasion. That code just means it doesn't see enough of a difference between the front and rear 02 sensor. if the car is full stock exhaust I would probably replace the rear 02 before I put a cat in it, or even put a rear 02 extension in. If it still throws the code after that, then sure, put in a good cat. All of the "high flow" and performance cats will give that code anyway, and then you will need the 02 extension.

    My 01 threw that code with every exhaust set up it ran, 2 different cats, rear o2 spacer, then the rear o2 elbow. With the cutout open after a "high-flow"cat it wouldn't do it for weeks, then I would close the cutout and it would come back in 10 minutes. Granted the 01 is far from stock.

    My 04 completely stock cleared that code with the rear o2 spacer and a tank of 109 and nothing else.

    o2 extension is like 10 bucks, rear o2 is like 60, high flow cat is around 100, stock cat is like 500. All of which may correct the code.
    Thanks. That's all great info. I will see if the code comes back. Car is completely stock, including the exhaust. Rear O2 is what I would start with. It's original and the car has 116K miles on it ...
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  5. #5 Re: 2004 GTP with P0420 diag code 
    The Blue One blueguy's Avatar
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    01+ cars need the rear o2.

    You can always get a 2.5" or 3" downpipe from ZZPerformance on the cheap. Bolts right in.
    Sold WBody's: '03 Blue GTP/'98 Green GTP/'98 Silver GT/'05 GXP
    '99 Chevrolet Silverado Classic Z71 4x4 - K&N Intake/Gibson Exhaust #TRUCKTHINGS
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  6. #6 Re: 2004 GTP with P0420 diag code 
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    Quote Originally Posted by blueguy View Post
    01+ cars need the rear o2.
    Silly question - whutchu mean "need'? I got a 2000 GT and haven't done anything to her yet. my 1998 GTP needs a new cat and was thinking about swapping down pipes for the up coming emissions testing. Does my 2000 have a rear sensor or you mean if it was disabled, it wouldn't do anything?? Thanks!
    Daniel - Code name ILMODC (il-maude)
    Current ride: 2000 GT, 1998 GTP (wifes) -
    Work in Progress: 1987 Trans Am – KnightHawk Edition (5.3 LSSwap)
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  7. #7 Re: 2004 GTP with P0420 diag code 
    Killa Bee Scottydoggs's Avatar
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    needs, means it actually uses it unlike the 2000 and older. you can still delete it via a tune. no tune, you "need" it. same goes for the older years, it needs to be tuned out to not see a cel, theres a few more steps in the 01-03 cars tune to delete it fully.

    98 Buick Regal GS, F body brakes, Caddy STS wheels, tinted tails L36 bottom end, lightly ported heads, 1.95 roller rockers, headers, gen 5 N* 3.0 pulley, FSIC, 42 lb injectors, a BrandonHall rebuilt trans, DHP tuned and AEM water/Meth injection https://goo.gl/gpV5kW
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  8. #8 Re: 2004 GTP with P0420 diag code 
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    Where do we get the spacer extension that was mentioned? Thanks

    Sent from my SM-G930R4 using Tapatalk
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  9. #9 Re: 2004 GTP with P0420 diag code 
    The Blue One blueguy's Avatar
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    Quote Originally Posted by therealtoddster View Post
    Where do we get the spacer extension that was mentioned? Thanks

    Sent from my SM-G930R4 using Tapatalk
    You can use an old Spark plug fouler trick to do it with a plug extension from the pipe...
    Sold WBody's: '03 Blue GTP/'98 Green GTP/'98 Silver GT/'05 GXP
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    '89 Ford Mustang LX Notchback - LM7 5.3, 4L80, 9", HX40
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  10. #10 Re: 2004 GTP with P0420 diag code 
    Killa Bee Scottydoggs's Avatar
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    help section or ask the counter guy, theres vids on youtube how to make the set up with the anti foulers, you need to drill one out. some show the box they come in.

    98 Buick Regal GS, F body brakes, Caddy STS wheels, tinted tails L36 bottom end, lightly ported heads, 1.95 roller rockers, headers, gen 5 N* 3.0 pulley, FSIC, 42 lb injectors, a BrandonHall rebuilt trans, DHP tuned and AEM water/Meth injection https://goo.gl/gpV5kW
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  11. #11 Re: 2004 GTP with P0420 diag code 
    SE Level Member therealtoddster's Avatar
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    Alright cool, thanks guys.

    Sent from my SM-G930R4 using Tapatalk
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  12. #12 Re: 2004 GTP with P0420 diag code 
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    Found a video. This will hopefully help future searches when people run across this thread.


    https://youtu.be/SLXb4fZx4QA

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  13. #13 Re: 2004 GTP with P0420 diag code 
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    My code is back. P0420. Tried to get that downstream O2 sensor out but it is rusted in there. Soaked it with PB Blaster, ran the car and tried taking it out twice with no luck. I'll pop an exhaust mount or something before I get that thing out. Any pointers on what works on getting those things out ??? Thanks ......
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  14. #14 Re: 2004 GTP with P0420 diag code 
    Turbo is the way to go. Fivefingerdeathpunch's Avatar
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    You need the red wrench.

    Gotta heat that ****er until it's glowing red with a torch, then get your wrench on it. Should come out like butter then.

    SMGPFC Member #1
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