srob817

I did some searching and I know it could be an Exhaust leak somewhere, cats, or Down stream O2's. I just wanted to get some other input before I go any further.

I purchased the car a few months ago and the check engine light was on and it had the p0420 and p0430 codes. The owner said it needed cats, i was skeptical since I could clearly hear an exhaust leak. So i bought the car and immediately replaced the 2 donut gaskets that were leaking exhaust. I cleared the codes and about a week later the codes came back. And from the looks of it both of the down stream O2's have been replaced thou I am not sure if they are good or not.

So what do yall think? Could I still have and exhaust leak, maybe further up after the Cats?? I cant hear or feel any other signs that it is leaking. I dont smell rotten eggs like the Cats are bad and what are the chances that they would both be bad at the same time???? Or could it be that both of the down stream O2's are bad!? The car runs great and has plenty of power, I average 21mpg so I guess thats pretty average. UGH not sure what to do.

Any help or info would be greatly appreciated. Thanks

brainsurge

You can easily check your o2 sensors with most obd2 scanners or obd adapters + smartphone (app is called torque), in the live view you can see the output voltages of all your o2 sensors.

Also have a look for your y-pipe, on my 01 gs 430 it is very rusty and leaks below the heatshields.

srob817

What kind of voltage am I looking for at the O2 sensors? Does it show all 4 sensors?

aneidiaz

Listen and read this very carefully to me bro. This is a relatively INEXPENSIVE repair if you just read my advice CAREFULLY. I had this SAME problem for almost 2 years before I changed it. Yes, I ended up buying new cats but ONLY because I wanted to. These cars are equipped with very durable cats. Half the time when they tell you its the cats , it isn't..... OK so this is what I did...

Go to www.sewellparts.com and sign up using your CL account information.

Look for all the gaskets to your car. Before AND after the cats. You will need to replace all the gaskets pre and post cat locations. You might want to buy double of each. Incase your manifolds have deteriorated where they meet that cats , put double gaskets there to insure extra seal. I did. It worked out marvelous. Also, I strongly recommend only buying OEM Lexus gaskets. WHICH IS WHY I SAID SIGN UP AT SEWELL PARTS , 100% TOYOTA OEM PARTS FOR LOW PRICES.

Lastly purchase all o2 sensors. Don't just change one. Change them all the first time. Get the whole problem out the way. I have a GS430 so I have 4 o2 sensors. Idk how many a GS300 has... Maybe 1 or 2. Pre and post cat sensors. Look up on eBay the Denso o2 sensors for your car. You'll be able to get them real cheap. I went with Denso. Good brand.

In total, all your looking to do is replace out all gaskets pre and post the catalytic converters. Also, replace o2 sensors. When there is air leaks, especially IF YOU CAN HEAR THEM, it most likely rotted gaskets before and after the gaskets. The codes your saying that your having all pertain to everything I told you. If done properly, all gaskets replaced should eliminate all air leak sounds and rough sounds from the exhaust. And new o2 sensors will be the icing on the cake. Better fuel mileage. The computer will read new sensors. All that good stuff. Should CLEAR CODES COMPLETELY IF INSTALLED PROPERLY. Hope this helped! I was in your predicament a few months back. I spent lots of money! But because I wanted new cats. Aside from that, when I looked at my old ones they were still in decent condition. I could've cleared the issue with just the gaskets and o2 sensors. Good luck! Try this inexpensive solution BEFORE resulting to new cats. Save your money and do it right at first and everything will come out fine. Have patience!

srob817

Thanks for the advice. I went ahead and signed up. I am really hoping it is an leak somewhere, especially since it says that both converters are bad. I looked on there for the manifold gaskets but they are showing on the parts list.

http://lexus.sewellparts.com/oem-cat...S300-2000.html

I found some felpro exhaust manifold gaskets on rockauto. i know you only recommend lexus parts.

aneidiaz

Bro only buy the OEM gaskets. Trust. They will hold up much longer. They aren't expensive at all. Like $5 each I think. Get the ones that go on the cat before and then the one that goes after. Get 2 of each for both sides just incase. And all new Denso o2 sensors. Watch how that resolves your problem. If it by chance doesn't, which I think it will, then you know for a FACT that it's the cats. But this is a very good inexpensive approach.

I seen the diagram you were looking at on Sewell. Try looking for the actual catalytic converyter. I mean the gaskets that go before and after the cat. I see you only have 1 cat. As opposed to my car that has 2. Yours is much simpler to fix. Should be an easy fix

islexusred

was having the same symptoms and same codes! it affected my MPG. i have Torque Pro ( the app ) on my phone that i used to clear the codes whenever they pop up, usually a couple of times between fill ups...the app is great to have but doesnt do a good job on pin pointing which sensor is bad. So one day i took my car to a local MIDAS replaced the 2 flange gaskets after the CATs, they also looked for exhaust leak which there was none! but when the used their advanced scan tool, they reported to me that 3 of my O2s are bad and they quoted me $900+ to replace them, and thats without labor...few days later i bought all 4 O2s off of amazon for about $ 190, took me an hour and half to swap'em out. MPG went up and no more codes...Dont let anyone talk you into replacing the CATs without replacing the O2s first.

srob817

Yeah I am ordering the gaskets and 2 down stream O2's first.

burny

What are the gasket part #'s? I do not see anything associated with catalytic converter. Would be in Exhaust section I would think. looking for these for 2002 GS300

srob817

It's the 17173 part. I called the dealer and went in and picked them up.

burny

let us know how the install goes.

aneidiaz

Great initiative taken my friend! I'm sure this will resolve your problem. I told you ☺. Try the inexpensive route first. It never hurts to try.

aneidiaz

Sounds like we all had this problem then lol. Except I was the only idiot who actually went and bought the new cats too smh. I don't regret it tho. I actually feel happy I did. The car has been a dream to drive after. Definitely more fuel worthy now. Despite it being a V8. But yeah, I'm pretty sure if I would've just changed my gaskets & sensors, the CEL would've turned off.

aneidiaz

This was my one cat after installation was all said and done...

srob817

Well I have some good news. This morning I installed the manifold gaskets and the 2 donut gaskets, cleared the codes and drove it over 100 miles with no check engine light!!!! So I took it to get inspected and it passed!! So I am good to go!

Thanks so much ANEIDIAZ! Total cost was about $50.00 for the gaskets from lexus.