Possible Cause

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:

Front HO2S malfunction
Open circuit between front HO2S and PCM
Open circuit between ignition switch and front HO2S
PCM malfunction
Poor connection at front HO2S or PCM connector
Short to ground circuit between front HO2S and PCM