zippinbye

I'm trying to resolve DTCs 420/430. Other threads indicate it could be catalyst failure, an exhaust leak (pre-cat) or 0xygen sensors, and specifically (mostly on other Lexus models) the post-cat O2 sensor. My 05 GX appears to only have pre cat O2 sensors. How can OBD II ascertain the efficiency of my cats without a post-cat sampling? BTW, these codes appeared simultaneously. The probability if bank 1 and 2 versions any of these problems happening at the same time seems remote. Am I missing something obvious?

RCsGX

If you dont have any post cat O2 sensors your GX has been hacked on. I have a '05 too and I have pre and post cat O2 sensors. The precat bungs are built into the header / manifolds. The post bungs are about where you 2 front doors are...obviously in the exhaust pipe.

zippinbye

Sorry RCsGX - I thought I was looking at cats, but must be resonators. So I stand corrected, it looks like O2 sensors before and after cats, so 4 sensors total.

But I still present the question: how is it possible for both sides to simultaneously go bad?

zippinbye

Since the title of my thread was based on a misunderstanding, I'm going to start another thread; not trying to double-post! Thanks for viewing!

diagtime

yes, both cats going bad is very common.

Catalyst efficiency is measured by many things, but mostly the ability to hold oxygen. the computer determines this by driving the engine rich and lean. to see the delay in the reaction of the rear o2's versus the front o2's. you basically just need to make sure all of the oxygen sensors are working and there are no exhaust leaks. there may be a little more to it than that but that's the basics. honestly, you probably have 2 bad cats.