P0420 Error Code

[Chris81]

Thought I'd post this here in the hope that it will get more views and hopefully a response.

Error code P0420 as we all know means 'Catalyst system below efficiency bank #1'

It's STILL coming up on my friends 2003 1.0 Yaris after a new cat has been fitted.

I've searched on here and the normal course of action is to replace cat and/or oxygen sensors, however my local garage (independent) said that it is a software fault and a Toyota dealer needs to fix it. I thought that was nonsense (but did wonder why the local garage wanted to turn work away). That lead me to here and I came across this post:

http://www.toyotaownersclub.com/forums/index.php?showtopic=115857&st=0&p=1047563&hl=p0420&fromsearch=1entry1047563

Basically it is the same as my local garage was saying - that P0420 can be the software that is at fault and showing up an erroneous message, and that the cat and oxygen sensors are not actually at fault.

Does anyone else have any experience of this? I've read posts where people have changed cat and sensors and MIL is still coming on with P0420.

Cheers.