Possible Causes

WARNING: This page is about a different car, the 2003 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Open circuit in White/Blue wire between rear HO2S 4-pin harness connector terminal "A" and PCM 30-pin harness connector terminal "1Y"
Open circuit in Brown wire between rear HO2S 4-pin harness connector terminal "B" and PCM 30-pin harness connector terminal "2H"
Rear HO2S malfunction
Short to body ground circuit in White/Blue wire between rear HO2S 4-pin harness connector terminal "A" and PCM 30-pin harness connector terminal "1Y"