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