Possible Causes

WARNING: This page is about a different car, the 2003 Mazda B4000, 2003 Mazda B3000, and 2003 Mazda B2300. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Corrosion or poor mating materials and wiring.
Damaged HO2S heater.
Damaged PCM.
Low battery voltage.
Open ground circuit.
Open VPWR circuit.
Shorts to VPWR in harness or HO2S.
Water in connectors (contamination).