Chem1

I've got a 2007 ES350 (142,000 miles) that has been throwing a P0420 code for the last couple of weeks. I've had my mechanic check the Catalytic converter (he says its not clogged up) and I've replaced both oxygen sensors. I clear the code and 2-3 days later (usually after short 5-10 min. drives) it returns. As a side note, the code never appears during longer (1-2 hour) drives. I am stumped. Can somebody suggest where I should turn next? Thanks!

FmrAccord

The catalytic converter is not working to the specs that the manufacturer expects. You will need to replace it.

oldgrump

This is one of the really good "fool the inept/inexperienced mechanic" codes. This code can be set by the ECM for one of several other issues other than the cat. Was the P0420 the only code? Did your mech follow the troubleshooting/code confirmation procedures in the Service Manual? Was any work or maintenance accomplished on the engine just prior to this issue? The following items need to be eliminated as the root cause BEFORE replacing the cat:
Intake air leaks (downstream of the MAF), vacuum leak (downstream of the throttle body)
Injectors or other fuel delivery issue
Injector leaks
Spark plug/s (or coil) causing an intermittent missfire

Chem1

Here is what my code reader gave me:
OBD2 - confirmed:
P0420 - Catalyst System Efficiency Below Threshold Bank 1
OBD2 - pending:
P0420 - Catalyst System Efficiency Below Threshold Bank 1
Engine:
P0420 - Catalyst System Efficiency Below Threshold Bank 1
I can't comment on what procedures my mechanic actually followed, all I know is he told me the cat was "ok" and that it wasn't clogged. I will check. And I will pass on those other suggestions as well. Thanks.
FWIW, I did change a spark plug and coil not too long ago (I can't remember which one for sure, but I am pr try sure it was in Bank 1) and last February I replaced the Bank 1 exhaust vvt solenoid.

oldgrump

Bank 1 is the aft bank so would you have not had to remove the intake plenum (the surge tank in Toyota-speak) etc. to access the spark plug and coil? Are you sure that you don't have any vacuum leaks related to the reinstallatiion of the plenum and associated removed parts after the plug and coil replacement? Check that the intake ducting at the TB is installed correctly. It appears that you've already been fighting a misfire issue. With the mileage that you had/have I would have probably replaced all the rear plugs and coils (along with the front plugs) at the same time you replaced the # 1 cylinder plug and coil.