Another P0420 thread (1 Viewer)

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Before I start I have done some research. I do have a question about something though hence why I'm creating this post. Bought the LX almost a year ago and drove it for several months with a leaking driver side exhaust manifold. It was leaking quite a bit. Never got a cel for it though. Finally got around to replacing it. When I took it off I couldn't get the old o2 sensor out of the manifold so I replaced that as well. Within a week I got the p0420 code. Since I already replaced the upstream sensor I went ahead and replaced the downstream sensor. Code went away for a few days then came back. I purchased this sensor from amazon but it is supposed to be an actual denso sensor. Both of them are. So, for the last few weeks the cel has been coming and going. Last night finally though I was able to hook my scanner up to it. The bank 1 down stream is definitely registering lower numbers than the bank 2 downstream sensor. My scanner also had a question mark next to the b1s2 sensor. I'm just really trying to keep from spending $1200 on a cat. It doesn't rattle or even have hardly any rust on it. I checked for exhaust leaks last night but couldn't find any. This afternoon my plan is to clean the maf, swap the downstream sensors and hopefully figure this out. I know there is the possibility of a bad gas cap as well. This rig has 450k miles on it so I know there is the possibility of a bad cat but is there anything else that I'm missing?

Thanks
 
If you got them from Amazon then I would roll the dice and order another denso sensor to see if that solves your problem. If so, then box up and ship back the defective sensor back to Amazon.

How are your smog laws in your area? You can do the spark plug fouler trick. about 20 bucks worth of parts and it should clear your 420 code. Some people have posted that it passed smog in their state. I actually have this done on my commuter honda element right now. Its been over 2,000 miles and no cel.


 
If you got them from Amazon then I would roll the dice and order another denso sensor to see if that solves your problem. If so, then box up and ship back the defective sensor back to Amazon.

How are your smog laws in your area? You can do the spark plug fouler trick. about 20 bucks worth of parts and it should clear your 420 code. Some people have posted that it passed smog in their state. I actually have this done on my commuter honda element right now. Its been over 2,000 miles and no cel.



I swapped the two downstream sensors last night and the error stayed on the driver side so it doesn't appear to be the sensor. There are no inspections of any kind in my state, yay for MS, so this may be my next step. I am still worried about it running properly though. There has got to be a reason that the sensor is reporting weird values. I may pick up a compression tester and check compression this afternoon. I would think if I had a head gasket issue though I would have a misfire code or be loosing coolant. Neither of which is happening. It's going to be slightly more difficult to do the non-fouler trick though given on my 2001 it's got the rear sensors with the two ten-mill studs holding the sensors in instead of it threading directly into the pipe.

Thanks
 
Check the exhaust flanges near there for leaks. My 80 used to wear those gaskets out every couple years and always throw a P0420 code. Easy to see where it was leaking on startup on a cold morning with the steam billowing out the leak. Gotta catch it quick, though, as it warms up down there in just a few seconds and you won't be able to see anything. Super cheap and easy fix if that's what it is.
 
Check the exhaust flanges near there for leaks. My 80 used to wear those gaskets out every couple years and always throw a P0420 code. Easy to see where it was leaking on startup on a cold morning with the steam billowing out the leak. Gotta catch it quick, though, as it warms up down there in just a few seconds and you won't be able to see anything. Super cheap and easy fix if that's what it is.
That's definitely a possibility. The code started after I put on a new manifold. I used a new factory gasket though. I can put my hand around the flange though while it's running and don't feel any gasses leaking. I guess if the leak is small enough though I might not actually feel it leaking with my hand. I got a temp gun last night and checked the temps going in and out of the converter and the temps matched up to the temps on the other converter so I don't think I have a clogged cat or anything like that which is good. I'm going to get some bg 44k fuel injection cleaner and run through it this weekend and see if that helps.
 
For tracking an exhaust gas leak the best bet is to stick a shop vac to the rear (duck tape it air tight) and use soapy water to monitor for a leak in the exhaust system. But at 420k miles thats a lot to ask for.. but Cat's may have not gone..

Also what are your o2 ST/LT trims reading? There could be really many reason's why you are getting this code.. and doing a step by step elimination is the only real way to t-shoot this properly.

Also at that mileage I hope the manufacturers stop monitoring for EPA s*it.. wouldn't that be good wishful thinking.. lol
 
I'll go through and look at my numbers again tomorrow. I took about a two minute long video with my phone, All of the numbers seemed in line with what they should be except the downstream sensor on bank 1. It would just go back and fourth between .015 and .035 which is considerably lower than it should be.
 

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