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:

Engine malfunction
Insufficient compression
Leakage in exhaust system
Open or short to ground circuit between right or left rear HO2S terminal and PCM terminal:
1. For DTC P0140: Right rear HO2S 4-pin harness connector terminal "A" and PCM 104-pin harness connector terminal No. 35 (White wire)
2. For DTC P0160: Left rear HO2S 4-pin harness connector terminal "A" and PCM 104-pin harness connector terminal No. 61 (Red wire)
PCM malfunction
Right or left rear HO2S deterioration
Right or left rear HO2S heater malfunction