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 front HO2S terminal and PCM terminal
1. For DTC P0134: Right front HO2S 4-pin harness connector terminal "A" and PCM 104-pin harness connector terminal No. 60 (Yellow wire).
2. For DTC P0154: Left front HO2S 4-pin harness connector terminal "A" and PCM 104-pin harness connector terminal No. 87 (Green wire).
PCM malfunction
Right front or left front HO2S deterioration
Right front or left front HO2S heater malfunction