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 cause are:

Connector or terminal malfunction
Open circuit in Blue/Black wire between ignition relay harness connector and right rear HO2S 4-pin harness connector terminal "C"
Open circuit in Yellow/Green wire between right rear HO2S 4-pin harness connector terminal "D" and PCM 104-pin harness connector terminal No. 95
PCM malfunction
Right rear HO2S heater malfunction
Short to ground circuit in Yellow/Green wire between right rear HO2S 4-pin harness connector terminal "D" and PCM 104-pin harness connector terminal No. 95