Possible Causes

WARNING: This page is about a different car, the 2003 Mazda MPV and 2002 Mazda MPV. However, it is still accessible from the selected car via links, so may be relevant.

Faulty right rear HO2S.
Open between ignition switch and right rear HO2S.
Open or short between HO2S and PCM.
PCM malfunction.
Poor connection(s) at right rear HO2S or PCM harness connector terminals.