ES 300 P0420 issue - fixed
 

Mine has the same issue, I just keep resetting the code every few months when the light comes on. I have put foulers on my 02 Sensors, and that fixed the majority of the issue (CEL used to come back right after it was cleared), but I fear this is only a stop-gap measure, and eventually I will have to bite the bullet.

Oops - I spoke tooooo soon. Lights came on, frustration :(

Besides using oxygen sensor fouler, what other options do I have?. Should I replace the oxygen sensor right next to (out) catalyst convertor?. I doubt the cat converter I got from ebay was bad as outside condition looked very good and no sound when shook.

Any suggestions?

Pull the O2 sensors and clean them with seafoam, them put them back in, clear the code and try again. It may be the cat fouled the O2s, or you may have bad cats on the manifolds. If so, PaceSetter makes a nice aftermarket set, you can replace both for around $500 if I recall correctly.

I don't have emissions tests in my county, so I may simply gut the cats (all 3 of them :rolleyes: ) and then use L-shaped foulers to remove the O2's from the exhaust stream completely.

I will try cleaning O2 sensor and give it a shot. Thanks for the suggestion.

Ok, here is the latest update - CEL P0420 went away after replacing the oxygen sensor (Denso 234-4137) which was right next to catalytic converter!.

For those interested in this CEL saga which was started about a year back with intermittent P0420 and would go away after few hundred miles. Stays off for few thousand miles and would come back on again. That interval started shortening to the point it would stay off few hundred miles and eventually stayed on. At that point, reset would bring back the code within 150 miles!.

4-5 months back I have had Lexus check for this. I gave them O2 sensor to replace as well. After few diagnostics, they returned the sensor and told me cat converter needs to be replaced. Have two other shops checked and they both reported the same.

As first post says, replaced the cat converter with used OEM one and light came back on within 100 miles. I tried a fouler approach, got one off of Amazon for $10 but its too big to fit in. At that point decided to give a shot to replace O2 sensor. Now with 200+ miles no CEL. I am pretty confident it would stay off.

There was a possibility just O2 sensor alone needed to be changed. But again I couldn't disqualify three different checks by different shops either. As ArmyofOne said, cat might have fouled O2 sensor. For those who get this code, its worth a shot to replace O2 sensor first especially if car has high miles.

I believe this car has two up stream air fuel sensors, replaced one at around 150k miles. Now I am at 321k miles.

Thanks MrBooby! Yeap, ready for State inspection :).

Heh, this is exactly what happened to me. I replaced all the 02's, and was motoring along for several months, code-free. Then one day, after about 1500 miles, the light came back.

The new o2's took 1500 miles to foul out. :(

I hope you have yours fixed though.