i have a 2003 mazda 6 s the car originally had a mis fire original plugs at 110k, so replaced them. runs great but now i have the p0431 code. replaced both o2 sensors reset light it is on again. do i need to replace the other o2 sensors or is the cat bad? and if so which one the one in the manifold or the one farther down stream?

 

Hey there buddy,
I had the same exact issue with my 03 6s. It turned out to be both Catalytic Converters on the manifold. they we're both pretty in bad condition. I bought the car used so it was expected. get them inspected check out the condition of them.. If it turns out to be the Cat's. What ever you do do not buy the stock Cat's from Mazda. they sell them to you for a bout $1,000 a piece. Ebay is the way to go. I purchased both and saved about $1,300.
Goodluck with the 6.