Hello, I'm Jason. I will do my best to answer your question quickly and accurately using the information you have provided.
The 421/431 are catalyst efficiency codes
The 1423 is related to secondary air low flow condition on the right(passenger) cylinder bank
you will need a pair of cats, and then need to do some vacuum testing to the right side combi-valve on the back of the cylinder head
cats as in catalytic converters?
the secondary air system is in place to heat the converters to operating temperature more quickly during a cold start.
make absolutely sure to use VERY good quality catalytic converters
otherwise the codes will come right back
Ok... Is the combi-valve easily recognizable?
let me show you a picture
they're about the size of a small yogurt container
ok. fantastic, that helps tremendously.
very glad to help
I'll get started. thanks again.
sure thing. Take care