I am checking out a friends mazda mpv for a friend etc.... Its throwing a PO421, which says its slow to come to temp for catalyst system.

I've heard people definatively say its a bad cat, and others claim its the o2. Can anyone provide any sort of insight or experience with this particualr prob?

Thanks in advance!

 

Van must be a California level emission to set a P0421. This code is indicating that the Right side bank front warm-up catalytic converter is below efficiency limit. This converter is built right into the right side exhaust manifold (Right side being closest to firewall, Left side is closest to the radiator). Have yet to see an oxygen sensor problem that would set a Cat efficiency code such as that. Make sure that there are no exhuast leaks from the manifold or outlet gasket area though as this has been know to throw a false code.
What kind of mileage is on the van? That part would still be covered under emissions warranty of 8 years or 80,000 mile so hopefully it is as a new one will set them back almost $900 for converter and gaskets, let alone labor charges as they are a bugger to get to.
There is a way to check yourself by monitoring the right downstream oxygen sensor signal voltage with a voltmeter if you are interested in trying to do that.

 

I think babyhuey nailed it. I had the same error code on my 2000 MPV (although I'm not in California). I have 170K miles on mine. Just took it to the dealer yesterday. Right exhaust manifold is cracked, which they said is most likely causing the reading. O2 sensor is bad too. Total bill for manifold & O2 sensor with labor is about $1,500. My manifold crack is bad enough that you can hear a "puffing" sound in the engine when it starts. Good luck with yours.