p0420 reoccuring code on 2003 Prius

Would it make sense to try and replace the oxygen sensors again, they are cheap? Other option would be is to have an aftermarket Cat welded in. Checking the vacuum diaphragm is also a good idea.
TomK

 

Which converter did you end up going with? Glad to here so far so good!
On this note, I was having the P0420 code come on every now and then until it was consistent around every 60 miles. I was ready to change the top O2 sensor and then decided to change the gas cap and have NOT had the code for almost 7,000 miles.
TomK

 

P0420 at 210k miles on my wife's 2003 Prius, changed cat converter (June 2014) with one from Eastern Catalytic (p/n 40544), had 4 months and 15k of trouble-free driving, and now (mid November, 2014)the code is back. Diagnostics for this fault are abysmal. I have a hard time believing it's sensors... but I could believe it's related to an incorrect ethanol level in the gasoline. Gonna try ethanol-free premium for a while and see what happens.

 

The last chronic P0420 I had experience with was due to excessive oil consumption . . . passing oil out the exhaust.

Bob Wilson

 

Bob, thanks for the thought on oil consumption, as I'm not the regular driver,
I'll have to monitor that level diligently and see if it could be a contributor.
What do you consider high consumption? The vehicle has 225k miles...

On my investigation vehicle:
I've put my OBD monitor on the OBD port, and
the upstream O2 sensor has a nice high amplitude swing, and
the downstream O2 sensor has very low amplitude swing,
so it appears as the cat converter and O2 sensors are working properly. Hmmmm.

I need to wait until it completes its readiness test again,
as I did a reset so my wife didn't need to drive with the CEL light on...

The witness states that the CEL came on as soon as the vehicle was started, so either the fault was declared then,
or it was declared in the previous drive cycle but the CEL light got turned on the next ignition.
My first thought was that it was the fault code that gets thrown when the butterfly valve in the throat
of the air intake system sticks (because it was a chilly morning, below freezing). But I was wrong...

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Not intended as an advert for any particular company (especially one specializing in cat converters),
but this website (link attached) does do what appears to be a reasonable job of describing a sequence
of tests to rule out possibilities of vehicle emission system failures other than the cat converter.

EXPLAINING TROUBLE CODES PO420 - PO421- PO430 - PO431

As well as this site (there are 4 linked pages, read them all):
What is Code P0420? | Code P0420

As a side comment, if anyone DOES decide that the single cat converter needs to be replaced on a Gen I Prius,
I will say that the Eastern Catalytic 40544 direct-fit is the low-pain replacement choice,
as the OEM cat converter is VERY CLOSE to the engine, and trying to fit a universal cat
into that space is a major endeavor... AND the 40544 it comes with a properly spaced port
to fit the downstream O2 sensor. I think it's worth its price in avoided pain and frustration.

To the original poster in this thread, although the California problem of cat converters is well known since 2009, apparently aftermarket cat converters are not available through retail channels in NY state, due to a change in legislation effective July 1 2013... now, why the legislature decided to make it tough for honest folks to make the correct repair is beyond me, but that's NY for ya...

 

Many of us go 5-10,000 miles between changes without having to add oil.

Bob Wilson