So I took my '95 LS400 into Autozone to have the check engine light looked at. Turns out after replacing O2 sensors its STILL bringing up P0420/P0430 codes which the guy there told me meant the catalytic converters are in fact going out. This would explain my car having an almost truck-esqe sound to it, wouldn't it? Because just driving around, it sounds like an F-250 driving by rather than a Lexus.

Any suggestions on what I should do for the cats? Aftermarket or OEM? Preferably not a total take out on my wallet

 

the OEM catalytic converters almost never go out, so if these are original to the car, I would try to hang on to them for dear life - they are fantastic! (and very likely not the problem unless the car has like 350K+ miles or has a bad rust problem from salt roads)

you need to take an inexpensive exhaust back pressure gauge and measure the back pressure, your "Autozone guy's" theory is a long way from definitive!

 

Well the car is barely pushing 170k so then that's out, and about how much is one of those back pressure gauges and what should I do once I get one?

 

you probably have some exhaust leaks turning your CEL light on and you probably didnt need the 02's either...

 

here ya go!

 

So it could possibly be an exhaust leak? Whats the best method to find the spot where its leaking from?
It also trembles kinda hard on start up and idles pretty low, usually midway between the 0 and 1 on the RPM and when Im driving it'll kick up but once my foots off the gas it goes riiiight back to being low

 

i suspect your EGR tube is leaking, causing the truck-like sound you are hearing. this is typical on our cars (exhaust leak) and will throw a code as you are describing. replace the EGR tube and go from there. as stated, it is highly unlikely the cats themselves are bad (if they are original).

 

lower EGR tube is a good candidate, since that is the most common place! (it cracks in the crinkled curved part)

I like to take a car to a good independent muffler shop, and they will put it on the rack and find the leak for you in 5 minutes and for free! Reward them with the job of fixing it, but if it is that lower EGR tube, make sure to use the genuine Toyota part.

 

a good link with pics:
https://www.clublexus.com/forums/ls4...-egr-pipe.html

 

P043/20? Check
Sounds like a truck? Check

9/10 exhaust leak(s). Look specifically at the headers themselves and the cat flanges. Could be the EGR piping though unlikely cause in this case unless it's a HUGE leak.

Pirate air is getting into the exhaust and mucking up your O2 readings with way too much oxygen, triggering the code you're seeing.