I have a 2001 mazda millenia with a trouble code P0421,
Definition: catalyst efficiency low-bank 1.
Explanation: the oxygen sensors monitor he catalytic converter ability to store oxygen.
Prbable Causes:
[ol][*]catalytic converter defective (failure pos due to #2,#3, or #4)[*]engine misfire or running condition[*]large vacuum leak[*]engin oil leakage into exhaust valve guide seals piston rings[/ol]
I see 2 O2 sensors before what I think is a "Y" shape catalytic converter and 2 more sensors leading back up to the back of the engine. Am I seeing correctly or is the converter further back.

 

what engine? 2.5, i cant help. 2.3, you have 2 precats, which are part of each bank exhaust manifold. the main cat is farther downstream. the reading you are getting is specific to bank 1(rear), post precat sensor. a common problem. log the date of the code, and erase it. it might stay off for a long time. if it comes back in a couple weeks, most likely the precat is dying. again, kinda common. not to bad to change, IF you have a lift. lots of stuff has to come off, like the exhaust system. takes about 3 hours with a lift and air tools.

 

My engine is a2.5L.....

 

same thing happened to mine,, i got 421 and431i took it to a garage and they said that 100% sure that my CATs are junk, kindda expensive because the CAT is part of the Exhausts manifolds, i couldn't afford to fix it yet, so i just punched through the CATs and fool the computer, so it won't set the check engine light, i put on about 500 miles so far so good.

 

What do you mean by "punch through the cat?" Search on line for your cat converter. There still exspensive but you will find some deals. At this time I'm on "autopartswearhouse.com and partstrain.com" but I'm still searching around. How many cats are on a 2001 millenia?

 

There are 3 Cats, 2 front ones are built into the Exhaust manifolds, and then1 rear main, i guess Mazda millenia has more problem w/the front ones than the rear main Cat.