My friend's 99 GS400 has the check engine light coming on and off. I made him take it to Autozone and had them pull the codes. He said the codes were P0420 and P0430 and bought some fuel injector cleaners because the clerk said it's probably moisture in the gas tank. He's also been using regular octane gas.

Do you guys think it's the combination of lower octane gas and moisture in the tank, or is his O2 sensors and/or catalytic convertors hosed?

 

I have the P0420 code as well, but I'm almost 100% positive it's because of the small leak I have at the flange right after the cat and before the O2 sensor. I can feel exhaust air when the car is running. Check to see if you have any exhaust leaks first.

 

DTC Description for those is Cat Deterioration.
It's either your cat's are gone, as in no more material inside. Or the post O2 sensors are bad.

 

Well, I haven't done anything to the car yet, but the CEL turned off on its own. Apparently it's intermittent. Is that still signs of the O2 sensors going out?

 

I have this exact problem and would like to know too...

 

I had this problem too so I took it in and they told me for new cats from the Lexus dealer were $1200 plus installation. So I when with aftermarket. But you should check for leaks and the sensors first they go from there.

 

To check if the O2 sensors is the problem, am I able to test the sensors themselves, or would I have to simply replace them?

 

I still think you should check for leaks, I put some exhaust pipe/muffler leak sealant on my problem and my CEL turned off for the first time in 5 days. I was thrilled lol.

$3.50 Autozone fix > $2700 from Sewell to replace a cat and all oxygen sensors.

 

Bump!!! I am having this problem. Code P0420. My dealer and an independent shop told me my Cats were bad. I have an 02 GS430 with 104,000 miles. I took it to a small muffler shop and they raised the car.I could feel warm air coming from the cats. Does this mean that the Cats are bad?

 

If you feel warm gas coming out, then it could be a leak. There are generally 3 things that will make the code pop up: bad cat, exhaust leak, or dead o2 sensor (after the cat).

 

Its a exhaust leak. Get it fixed and ur set.

 

There is a leak! Jeez, does anyone even read the thread before they post in it?!

 

no, im telling you people dont read anymore, they just skip to the end and post up a question. MOST of the time its something that has already been asked or talked about but they are to lazy to read it

 

I have a 2003 GS300 that is throwing this code (P0420) as well. It's only got just under 80K miles. Does that sound a little low for the cat to be going out? Is this more than likely a bad O2 sensor, with that few miles?

 

You should run to the Lexus dealership quick before you hit 80,000 miles because the Cats are factory warrantied up to that point. Lexus will most likely pull that code and say that your cats need to be replaced and do it for free!! You're fortunate!!