P0420 again error code 1 year after replacing cat

Oil blowby is contaminating the cats. Original cat is 100% better quality than the aftermarket cat. My guess is your after market cat is contaminated now also.

 

In general, having a cat fail was more common years ago back when gas was still high in sulfur. Switching to low-sulfur here in the United States was definitely helpful to emission-system longevity, as well as simply being cleaner in the first place. I'm not sure about Australia's gas status or how long the sensor itself will last. Burning some oil now complicates the situation too. Technically, you can still drive with the cat not cleansing as well anymore. It's more a matter of duration of ownership you are hoping for still and if there are inspections/regulations about needing replacement. Good luck.

 

A litre every 10,000 km is nothing in the scheme of things. A P0420 does not necessarily mean your cat is bad. There can be upstream reasons for the cat reporting that performance is compromised.

A good diagnostician should be able to work out what the issue might be. The hard part is finding a good diagnostician, sad to say.

 

In my experience PA0420 error code in Gen1 and Gen2 is primarily caused by people who take their cars to oil change places that don't care about adding a precise amount of oil and chronically overfill the engine a bit above 4 quarts. Over time these engines run way dirtier and foul the exhaust system. There's also a history in Gen1 Prius of doing a tune up (spark plugs, oil & air filter, throttle body and mass air flow sensor cleaning) to eliminate PA0420 error code. I've found this works with Gen2 as well, at least for a while.

I'm not a fan of changing the Cat's O2 sensor or changing the Cat until all the other less expensive options are addressed...

 

Sure wish there was... I've never run across someone doing in-depth diagnostics on Prius exhaust system so it's less of a guessing game to fix. If you can find someone who can do that, please let us know so we can help others do the same.

 

Here is my P0420 checklist.

Is the engine running properly? - No misfires, even compression, no excessive oil consumption.
Are there NO exhaust leaks from the cylinder head down to 18" past the downstream sensor - ZERO -NONE (not even a pinhole)
Is fuel control good? Both O2 sensors respond full rich to full lean, and fuel trim data looks "reasonable" (under +/- 10%)
Are there any relevant recalls or TSB's? (technical service bulletins)
If all that passes then I recommend replacing the cat.

If your car does need a cat AND only the "upstream" cat is monitored AND your only concern is the P0420 THEN you could replace only the one cat. BUT I would expect most assemblies for the Prius would include both cats anyway.

I don't know if the second cat is a NOx reduction cat or if it's a second oxidizing cat to try and scrub more HC/CO for cleaner (PZEV) tailpipe emissions.