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.

Engine compression low.
Engine malfunction.
Exhaust system leak.
Front HO2S deterioration.
Front HO2S heater malfunction.
Open or short circuit between front HO2S and PCM harness connectors.