I live in NY, and I have 2004 RX330, with approx 130,000 miles, and my check engine light has been on since about a week after my 2010 inspection. My local Lexus dealer says its a P0420 code, Catalytic converter problem, and quoted me approx $1500 to fix and inspect my car. I talked to a friend who works at a local garage, and said to buy the cat from NAPA ($321) and he would put it in for 2.5 hours labor, and $200 more later, the CEL is still on, and my vehicle still can't pass inspection. All sensors are reading within specs on his Snap-On $10000 OBD tester, and it makes no sense to him, his shop owner, or me for that matter. If all the sensors are within spec, how is it throwing the CEL and not resetting the monitors, so I can at least get it inspected. I don't really care that the CEL is on, and my fuel mileage hasn't changed a bit, and it seems to run just fine. I just need to get my vehicle inspected, and NY is a pain in the a$$ about it. Any ideas ??

Just curious, did he reset the ecm after completing the work? if so,maybe it`s something stupid like the heater circuit in one of the airflow sensor`s acting up.

I had the same code appeared on my '06 RX with both the check engine and the VSC light on. What I did was I pumped 93 gas for a while and reset the battery every time it came on to clear the warning lights. In normal driving condition, the lights will not come on when I drive under 60 mph and will reappear when I hit the highway and drive for 3-4 hours nonstop. I had to reset it every time it happened, but oddly for the last 3 months it just doesn't act up anymore. I suspect it might have been the 87 gas plus a bottle of fuel injection/cleaner I put in then. So low quality gas can cause it? I've gone back to use 89 gas and everything is good for now.
***Also, i think it helps when I cleaned the fuel sensor.

Mostly the 87 octane gas because it burns richer by trying to keep it from pinging. And it ends up with unburned fuel in the system which is hard on the cats. You should be okay on the 89 octane, I really dont think it will cut too much timing etc. on that grade.

Ask them to reset the ECU. Drive 50 miles and if the light does not come back, go for the inspection.

Change the sensor after the cat ,and it'll go away. The reason the scanner says the sensor is fine, is because it may be a intermediate issue.

Have you figured this out? My light has been on for 20,000-30,000 miles now. Pulled spark plugs last week and they had a bit of white residue on top. Friend said it comes from the CAT not working properly - too much O2, not enough fuel.
No change in MPG, should I deal with this issue?

It can come with weather. I noticed vehicles in Mexico City getting this under very hot conditions (it's Mexico, but that's not normal for Mexico City being 7000+ feet in elevation). Under those conditions, a P0420 would get thrown. The sensor is usually the one I would toy with first before playing with the cat. Keep in mind, all sensors can degrade over time.

My 2006 FWD RX330 had this same code and I wanted to raise this thread from the dead in order to let everyone know that replacing the back-most 02 sensor, part number Denso 234-4509, resolved my check engine light. I purchased the part on Amazon (Denso is the OEM supplier) for ~75$ and replaced it myself. Extremely straightforward fix for any DIYer. All you need is a 22mm (maybe 24mm) open ended wrench and some pliers and it unscrews. My RX had ~179,000 miles and has been check-engine-light-free for 3000 miles.

pull out the maf sensor. blow some compressed air into it and clear the codes and preform 2 drive cycles. I have seen the MAF sensor triggering this code.
http://www.totalcardiagnostics.com/s...i-drive-cycles

your year is not listed but its still the same.

Kind of old thread...2011...

I had problems with 420 error code as well. Until I switched to a higher quality (not grade) gas. Forget about replacing CAT converter - using a low quality gas or higher ethanol concentration gas can trigger this error code.

Yeah, but still helping in 2019. : )

I had the same problem but only while travelling on the highway.every year 750 miles to NC each way. It only happens when I got crap gas from a gas station off an exit on the highway. When I get home I run Mobil 93 octane and problem goes away. I've been trying to figure this out too.

Thank you everyone for sharing your experiences.

Hey - Have you ever changed the oxygen sensors? I just got this code and was about to change them since they never been changed. P420 with Lexus in my experience has always been oxygen sensors even though it says catalytic converters. People freak out when they see catalytic converters when really the reason the code states cats is because the oxygen sensors aren’t giving a proper ratio to the cats causing them to throw the code