Possible Causes

WARNING: This page is about a different car, the 2003 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

Possible causes are:

Connector or terminal malfunction
Left front HO2S heater malfunction
PCM malfunction
Short to power circuit in White/Green wire between left front HO2S 4-pin harness connector terminal "D" and PCM 104-pin harness connector terminal No. 94