Help with P0420 Catalytic Converter and Y-Pipe Leak
 

Hey guys,
Need some help please. I’ve got a 2006 SC430 with 80k miles that I’ve had for 9 years without issues. I just got the CEL light on and the car sounds kinda loud now (rumbling exhaust) all week.

Brought the car to Lexus. They said I need the following fixed:
- two oxygen sensors
- leaky y-pipe
- catalytic converter bank 1 replaced
Quote was for about $3800! Help!

Im not mechanically inclined and never had a single issue with the car. Always doing routine maintenance at independent mechanic (who refused to do these repairs and suggested I take it to Lexus, I guess he’s not very good)

Any suggestions on what to do? I’m located in NY.
Could the issue be just the leaky y-pipe that’s causing the diagnostics to show failed catalytic converters and oxygen sensors? Lexus charged me $180 for the diagnostics...

coukd I take it to a muffler shop for them to weld possibly and fix the y-pipe that way and would that resolve my issue?
I really can’t spend that much money on it right now $4,000 is a lot considering the value of the car. But it’s in great shape otherwise. I’ve taken good care of it. Not really sure what to do at this point.
can you guys give me some advice please? Would really appreciate. Love this amazing forum.

Its not a cat issue. There is no way your cat has gone out with that little mileage unless you are the unluckiest person on the planet or you have a major oil leak inside thats sending oil out the back.

First off, do a forum search for the P0420 code you called out. Read everything about it. Have the obvious leak issue repaired. Test your O2 sensors and have them changed as necessary.

Keep us informed as you go along so we can help if possible. Good luck.

Paul

There's also a video on YouTube by a guy named "Scotty Kilmer" and he just recently did a video called " This illegal mod will make your car run better." I strongly recommend you watch that too and use that information to help you fix this issue.

Paul

Thanks guys. It’s actually my parents car. They barely drive it!
Im not sure how to test the O2 sensors or where they even are located. Is this something simple to do?

Ive never looked underneath a car...
Will try to find a muffler shop open tomorrow to see if they can just find and weld the leak on the Y-pipe and report back. Hopefully that will fix it?? That would be amazing. Not sure why the dealer would say to do all these things if it’s only one issue? I’m laying for all this for my parents so hopefully this can fix it?

Ive read pretty much every thread that has p0420 on this sub forum. I think only one post had someone test it and posted graphs. Seemed over my head

Well, you reasons are honorable but like anything else in life, knowledge has to be earned through hard work. We're here to help but you are going to have to find a good reputable mechanic or learn about how to become a decent mechanic. 😉

Paul

The loud noise probably caused by the leakage from your muffler. I don’t think it’s your catalytic converter.
Your DTC definitely your O2 sensor(s). Any decent mechanic should be able to replace it (them).
I have not have to replace my SC sensors so I don’t know the difficulty level. Someone might chime in.

Hey Maxse, when the mechanics have the car up on the lift for the leak repair, have them show you the O2 or oxygen sensors located in front of and behind the catalyst converter. Make sure the wires are intact and undamaged.

As far as gaining knowledge goes, the university of YouTube is the best place to go. Just go into search mode and type "what is an O2 sensor" or "How do I safely lift my car." You will be amazed at the amount of quality material on just about any subject you can think of. "How to change oil" for example. It probably won't be Lexus SC430 specific, but you will get the idea.

And never forget about looking at the Owners Manual which should be in the glovebox. It is filled with all kinds of information that is specific to your car and should be your very first place you go to for information like jack points, how to properly tow your car, fuse locations etc.

Paul

here are my two cents on the subject...refer to pics below.
i circled what I think you are referring to as the Y pipe.
if you have a leak at the Y pipe, it is NOT causing the P0420 fault code because there are no sensors down stream.
P0420 Left drivers side
P0430 Right Passenger side
these faults are caused when the results of the two O2 sensors are out of tolerance.

the Lexus dealer is “shotgunning” the problem by replacing all the parts which would normally cause the fault. Two O2 sensors and the Catalytic Converter.

As others have said, check wiring first, also check for exhaust leaks closer to the engine...they can cause the P0420 fault.
If you find a mechanic who can properly read the outputs of the O2 sensors, then he will replace the correct part.

if it were me, I would replace the two O2 sensors on the drivers side first, then go from there. That is assuming no broken wires and no other exhaust leaks.

let us know how it goes!

https://cimg9.ibsrv.net/gimg/www.clu...3f0858e46.jpeg
https://cimg2.ibsrv.net/gimg/www.clu...210bfa0d7.jpeg

I like this guy...

Guys, thank you Sooo much!
i was at the shop this morning just came back.
muffler shop said to try to fix the y-pipe and hopefully that was it but they had to replace the y-pipe $160 and welded it into place. I reset the code by disconnecting the battery terminal. Drove 30 miles already and no codes!
the car is also quiet again and doesn’t rumble any more like it used to!

i had him show me the O2 sensors. They looked like they were in good shape. But I only saw 2 of them the left side and the right side...

hopefully this means the issue is fixed.
I had it fixed at a muffler shop near a junk yard so I doubt they could test the O2 sensors. Didn’t see any sophisticated equipment. It was one of those shops where they only do muffler work.
Any thoughts? The “y-pile” was the part closer to the front of the car (as in the picture you attached). Thanks so much guys! Hopefully that did it!

I had a leaky gasket in my exhaust system and had similar errors as you.

If you want to a try a free diagnostic, jack up the front of the car and make sure it is secure. (You can also use car ramps)

Start the car and just crawl under and listen or feel for air sucking along the exhaust pipe. Start where the pipe comes out of the engine. Check both sides. It is MUCH MUCH MUCH better to do this when the car is cold.

I would be willing to bet it is where the two pipes meet (Catalytic convertor to pipe) If that is the case, see if you can tighten those nuts.

If you do need a new catalytic convertor, you can buy one off ebay. I bought one, but it turns out when I went to put it on, I found the leaky gasket. Fixed the gasket and all is well.

If you want to make me an offer for my unused cat, you can PM me.

Thanks!
Btw, if it actually wasn’t fixed, how much can I expect to drive before the CEL comes back on. I drove 30 miles already with no issues.

Thats hard to say. There are several other things you can do and should be part of a tune up.

Number 1 is to replace the PCV Valve on your next oil change. I doubt if it has been changed before and would be time. Its cheap. Less than $10.

Number 2, get a can of Mass Airflow Sensor cleaner. Clean the mass airflow sensor.

Number 3 is to check the air filter and change it if it is dirty.

Those are a few of things "you" can easily do with a little research on YouTube showing you what you should expect. Glad the problem seems to be taken care of.

Paul

I am not sure if it’s a mileage or a engine on off cycle thing. I think it’s time running so it can sample the system long enough.
if you drive everyday and the problem still exists, I would guess in two days, you will know.

I had the same fault code on my 2001 with 123000 miles , code kept coming back every 200 miles or so , then it got a lot more frequent.
I then noticed an exhaust leak from the cat to the start of the exhaust , there was a gap where the two parts join , i put some gun gum (exhaust paste ) in the gap and the fault stayed away .
After a while i got the fault code back , when i looked under the car i could see another slight gap had appeared from the cat to the exhaust but in a different place , so i sealed it again and the code went and stayed away .
I believe i have a failing gasket between the cat and the exhaust , i will get i replaced eventually , they are like a large washer and just seal the two parts .
Think this all started when i had a custom exhaust made from the cats back .