P0420 code

[redsoxsstink]

2002 camro 3.8, pacesetter headers, stock cat/exhaust

i am throwing a P0420 code (catalyst efficiency bank 1) i know it could be either o2 sensors or my cat going bad. i need to pass inspection next month so im trying to figure out what is the easiest way to fix this and still have the car ready for me to drive everyday.

if i were to replace the o2 sensors which would i do? upstream or downstream? or both?

if the sensors dont fix the issue, i can try a o2 sim, which would be the best to go with?

o2 sensor spacers? opinions? do they really work?

im thinking its my o2 sensors is the problem, the cat isnt glowing or making noise so i dont think it would be clogged. i know it still could be bad even without these things. im also getting junky gas millage, i normally get 30mpg highway i just averaged 21 over course of 200miles.

i rather fix it the right way and no use sims and stuff but im not opposed to it just to pass inspection. just looking for some guidance before i just start throwing parts at it

[greenformula92]

Sounds like the O2 is bad. I believe you will want to change the one in the header. The coating on the headers may have ruined them.

[Rob WS6]

Usually that code would be from a bad REAR O2.

[Dudbird113]

Pull the rear o2 and see wat she looks like. I usually pull mine every once in a while and burn the junk off with a map gas torch

[WildBillyT]

Usually that code would be from a bad REAR O2.

My thoughts as well. Pull it and replace it. Rock auto has good prices on O2s.

[The Fixer]

From my OBDII Jeep days, the P0420 code comes up usually because of either a lazy rear O2 (as mentioned) or the cat itself. On my old '01 Cherokee, I lucked out - it threw the P0420 code, I changed the rear O2 sensor, and the code came back. I happened to be just under the 7/70K emissions warranty and Jeep replaced the cat. Good thing, because on the XJ the two pre-cats are together in a y-pipe just below the main header and it would have been crazy expensive to replace on my own dime. :)

[L695speed]

I'm fighting this same code on my own daily driver. Vehicle being a 2000 Subaru Outback 5MT. I had it when I first got it, it needed a new exhaust anyway; and even after replacing the whole exhaust four years ago from the Y pipe back I was still fighting the light. With the Subaru, there were two sensors one before the cat, and one after. For me the light is intermittent it comes on, goes away, then comes back and stays on for months on end.

I've given up trying to fix the light, but with the subaru there are conflicting reports. First its automatically the cat, and mandates replacement, or its one of the two sensors. Or its something else throwing it. I have a friend who traded his WRX in instead of fighting that light. I am going to do the same thing before my emissions is due in August. Granted I have a laundry list of other problems. And with the exception of oil changes I am not fixing anything else as its become more of a money pit than even the two Pontiacs.

I would try the sensors before you do anything else, word on the forums is to go with OEM sensors for this. The aftermarket ones won't do much to fix the problem in the long run. If the sensors don't work, its time to look at the cat. Cat doesn't fix it...well, I hope you have better luck than I did. As for the emissions light...I cheated both times to get it through. Cleared the computer, drove it 50 or so miles and then took it to emissions, somehow their equipment read the OBDII both times. I may have gotten lucky though.

[WildBillyT]

I'm fighting this same code on my own daily driver. Vehicle being a 2000 Subaru Outback 5MT. I had it when I first got it, it needed a new exhaust anyway; and even after replacing the whole exhaust four years ago from the Y pipe back I was still fighting the light. With the Subaru, there were two sensors one before the cat, and one after. For me the light is intermittent it comes on, goes away, then comes back and stays on for months on end.

I've given up trying to fix the light, but with the subaru there are conflicting reports. First its automatically the cat, and mandates replacement, or its one of the two sensors. Or its something else throwing it. I have a friend who traded his WRX in instead of fighting that light. I am going to do the same thing before my emissions is due in August. Granted I have a laundry list of other problems. And with the exception of oil changes I am not fixing anything else as its become more of a money pit than even the two Pontiacs.

I would try the sensors before you do anything else, word on the forums is to go with OEM sensors for this. The aftermarket ones won't do much to fix the problem in the long run. If the sensors don't work, its time to look at the cat. Cat doesn't fix it...well, I hope you have better luck than I did. As for the emissions light...I cheated both times to get it through. Cleared the computer, drove it 50 or so miles and then took it to emissions, somehow their equipment read the OBDII both times. I may have gotten lucky though.

If you have to go in August, that's a good thing. Cat would get nice and hot.

[redsoxsstink]

Thanks for giving some insight, I want to just do the sensors and see where that puts me. My father just wants to throw the cat it and call it a day.... To me that doesn't make sense if changing the sensor is so easy. Plus it's easy for him to say when he's not buying the parts.

Also the headers were used so the coating was burnt off already

[WildBillyT]

Thanks for giving some insight, I want to just do the sensors and see where that puts me. My father just wants to throw the cat it and call it a day.... To me that doesn't make sense if changing the sensor is so easy. Plus it's easy for him to say when he's not buying the parts.

Also the headers were used so the coating was burnt off already

Change the cheap parts first.

[sweetbmxrider]

Sounds like the cat to me. A bad rear o2 won't affect your gas mileage. Winter gas will bring it down along with this cold weather but that's a good drop so I'd suspect a larger issue. Cheapest way would be to get an o2 sim to get through inspection but you'd eventually have to replace the cat. You can also clear the code and then set all monitors but the cat monitor. State inspection allows for one monitor to be not set. If the monitor isn't set, you won't have a code, you win a sticker. How many miles are on the car?

Federal emissions warranty is 8 years/80,000 miles, not 7/70.

[redsoxsstink]

Sounds like the cat to me. A bad rear o2 won't affect your gas mileage. Winter gas will bring it down along with this cold weather but that's a good drop so I'd suspect a larger issue. Cheapest way would be to get an o2 sim to get through inspection but you'd eventually have to replace the cat. You can also clear the code and then set all monitors but the cat monitor. State inspection allows for one monitor to be not set. If the monitor isn't set, you won't have a code, you win a sticker. How many miles are on the car?

Federal emissions warranty is 8 years/80,000 miles, not 7/70.

just about to roll 95k

[sweetbmxrider]

Yeah, I'm thinking cat. You could go cheap and just swap the rear o2's and see if the code follows or stays. You could also try knocking on the cat and exhaust and listen for any rattling of loose honeycomb.

[coolmanvette75]

Not sure if you know anyone with a decent scan tool. If you do you can clear the code run an O2 datastream. Monitor all the sensors and when the light pops on check the stream to see if a sensor is dead or reading low. Sometimes lazy O2's can be tricky. They will work fine for a little while, then not work at all, then magically work again.

[L695speed]

Yeah Bill, I think the hot weather may have been a contributing factor to me getting through emissions. However, the light still comes on even in hot weather. So in my case its luck of the draw. With the number of other problems (some are pretty major), I am just gonna drive it til I don't feel comfortable with it anymore, then park it til I can trade it in. My car has 228K miles on the clock, and its 13 years old. I can buy another one for not much more than the cost of the parts I'd need to bring this one back to what it was.

Anyway, if the OP is intent on replacing sensors, I'd go OEM to be on the safe side and hope it works out for you. Agree with others, worst case you'd have to replace the cat. A number of forums swear by the OEM parts being the make or break when it comes to fixing something like this. Aftermarket parts as we all know are usually nowhere near what the OEM stuff is when it comes to sensors.

[maroman88]

my TA has this problem, swapped out the o2 with no luck... guna have to replace the cat

[redsoxsstink]

Def gonna get acdelco sensors when I do replace them. Thanks again for the help

[GhostDakota]

FWIW: My P0420 code has been on and off for over a year. The shop that tunes LS1's by me hooped it up to their laptop. Driver side, front o2 is bad. Ordered a new one from Rock Auto, will see what happens when I install it. Just got it in the mail today.

[Rob WS6]

My Tahoe had that code, I replaced the stock 200k mile cats, and the code is still there, which obviously points to the sensors being shot. Should have gone that rout first, much cheaper. :lol:

[sweetbmxrider]

What converter did you replace it with? This is where quality really matters. Most cheaper aftermarket cats will not correct the 420 code or do so for only a short while. I'm not saying that is your issue, just my own observation.

http://www.obd-codes.com/p0420

Overall probably the biggest mistake vehicles owners make when they have a P0420 code is to simply replace an oxygen sensor (H02S). It is important to do proper diagnosis so you're not wasting money replacing parts unnecessarily.

[Rob WS6]

Dont remember what brand, was an entire y-pipe assembly, OEM replacement. Overall though, was a waste of money, thats what it was. :facepalm:

[redsoxsstink]

well my rear 02 sensor didnt fix it. junk gas millage and CEL still remain... time to weld in the hi-flow i have laying around

[Mezzy]

My gas mileage improved dramatically when I replaced both my O2s

[Grifter]

I've never replaced anything but a cat for a P0420...

[redsoxsstink]

o2 spacer didnt help enough to get through inspection, welded new cat in tonight. the old one looked clogged, hopefully this can get me through inspection.