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