My 2002 Stilo 1.6 gave me an unexpected error code yesterday, the P0420, i.e."Catalyst System Efficiency Below Threshold (Bank 1)". I connected to the ECU with MultiECUScan and deleted the error code, and it hasn't reappeared yet (though I've only driven about 40 kms or so since then).

Reading old threads on this board, I saw that (if I understood correctly) this error code is triggered if the post-cat lambda values too closely follow the oscillations of the pre-cat lambda values, and that ideally the pre-cat lambda value should oscillate whereas the post-cat lambda value should be relatively stable.

I decided to use MultiECUScan again, so I started the car and drove it a few kms for the engine to reach working temperature, and then I studied the pre-cat and post-cat lambda values of the engine when it was idling. The results (screenshot and CSV) are attached. From what I could see, it does indeed look like post-cat lambda graph is oscillating, which to my untrained eyes doesn't look promising.

Am I looking at a new catalyst, or could there be another cause of this problem? And am I right in thinking that the values don't look good?

yep... been through all of this with mine (see here for more info). changed lambdas etc refused to believe it was a cat... finally bit the bullet but i found a good one at a scrappys for about £30... i also put the lottery on that day feel free to scout 'round some more and post back with any ones you find. you will need Exhaust Manifold gasket and a Manifold to 'Downpipe' gasket. Cats now have to be 'type approved' and stamped accordingly iirc. im pretty sure that you cant buy non type aproved cats anymore but feel its worth the mention. it basically tries to stop you bodging cats on that werent designed for the engine but im 99% certain that even 'Generic/Universal' cats are now type approved.

Cats2Go don't list them anymore/their website database is down... might be worth a ring or an email i seem to remember them being cheap-ish around the £100 mark.
Cats2U £111+P&P, they price match, and apparently their stock is Stainless steel, 2yr warranty etc. they also have part numbers from different companies so you can cross reference
Autobits £105+P&P Stainless, 2yr, fitting kit available for £0.88... don't know what it consists of though.


lambdas last from around 50,000 to 110,000 miles but if your changing your cat you should really change both lambdas too. I suspect the reason my cat blew itself to pieces was because of a dodgy lambda. i think it was incorrectly tracing and therefore sending incorrect fuel level signals to the ECU. the ECU would then turn the fuel up and it would run rich. excess/unburnt hydrocarbons in a cat will kill it. ill see if i can dig out the picture of my damaged monolith.
Aim for as good as you can afford. imho id be going for NTK (same company as NGK spark plugs) or Bosch!

The pre and post lambdas are different. These are the cheapest NTKs i found when i was looking, although prices may have changed
NTK OZA659EE49
NTK OZA659EE31

OEM Bosch part numbers are:-
4wires, 630mm Lead 0 258 006 193
4wires, 430mm Lead 0 258 006 376

EDIT:- my cat damage looking up from the 'downpipe' you can just see the post cat lambda threads in the pipe out of focus at the bottom of the picture.