I have a 2001 Millenia 2.5 and just got a diag at the Mazda dealer. my car came up with P0421 and P0423 and was quoted by the dealer $1100 for parts and $500+ in labor. Does anyone know what these codes mean and what alternatives I can take to fix these issues. I would have had the dealer fix the issues above but they would not guaranty that my problems would be solved

 

P0423 is not a valid code number. Does not exist for 01 millenia.
The P0421 is a Warm-Up Catalytic Converter Efficiency code. It means that the right bank catalyst, located in the righthand side exhaust manifold, has been tested by the PCM and and was found to be getting weak and not cleaning the exhaust of harmful emissions as well as it should. This can sometimes be a glitch code but not often as it needs to fail the testing procedure two times in a row to turn the CEL on. What is the mileage currently on the Millenia?

 

My milage is @ 102063. I double checked my paper work from the dealer and here is what they described."customer reports check engine light stays on. perform system checks, codes P0421 & P0423 in system, needs 2 catilytic converters replaced, may have other problems. Customer advised" I am going to do a full tuneup being that I need one anyway and have my throttle body cleaned but it seems as though I might need to replace the cats unless there is a chance that it's the sensor but im not sure.

 

Was hoping you would be under 80k as would then still be under emission warranty which would cover the converter replacement.[:@]
Maybe they just had a typo on the P0423. P0431 would be the code set for the left side cat converter below efficiency limit. Tune and throttle cleaning ok but will not help with the CEL and codes. Faulty sensors very rarely cause those two codes to be set. O2 sensors have there own group of codes that get set if a fault is detected in their operation.
You could just try clearing the codes and see how long it takes before they return if they ever do. Is not going to hurt anything unless you need to pass an emission inspecting soon to get you registration renewed.
If the converters are needed you can get replacement OE parts at www.onlinemazdaparts.com for $438.30 each instead of $547.88 each from your local dealer.