2006 Mazda 6S: P0421 Warm Up Catalyst Efficiency Below Threshold

Neo Fender

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2005 & 2006 Mazda 6S
I understand this is a common code and that it often points to a bad pre cat but I also read that beginning with the 2006 model year, the pre cat(s) were eliminated and the O2 sensors were relocated. Appreciate any good info on what the problem may be.
 
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