Thread: 2004 GTP with P0420 diag code

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 !!!

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 .....

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.

Originally Posted by 01Blackgtp

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 ...

Where do we get the spacer extension that was mentioned? Thanks

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Alright cool, thanks guys.

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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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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 ......