DTC P0138:00, P0158:00, [MZI-3.7]: Notes

WARNING: This page is about a different car, the 2009 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

DETECTION CONDITION & POSSIBLE CAUSE

DTC P0138:00 DTC P0158:00	P0138:00: HO2S (RH) circuit high voltage P0158:00: HO2S (LH) circuit high voltage
DETECTION CONDITION	P0138:00 indicates that HO2S (RH) circuit high voltage P0158:00 indicates that HO2S (LH) circuit high voltage The HO2S signals are monitored for an over voltage condition. The code is set when the HO2S signal voltage is 1.5 V or more.
POSSIBLE CAUSE	Short to power supply in the wiring harness or HO2S PCM malfunction
Courtesy of MAZDA MOTORS CORP.