Code Definition

P1138 — HO2S Bank 1 Sensor 2 Signal Above 0.45v (A/F Ratio Too Rich)

System: Powertrain  |  Type: Manufacturer-Specific  |  Make: Mazda

About P1138

P1138 is a manufacturer-specific powertrain diagnostic trouble code recorded for Mazda vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P1138 is stored, the module has detected a bank-specific fault, meaning it applies only to one cylinder bank of a multi-bank engine; and a sensor-related fault — the reported value from the named sensor is out of spec.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

P1138 is a manufacturer-specific code. The same code string on a different make may have a different meaning. Always confirm the code's definition against the vehicle's service information for the specific make and model under test.

Code Information

The Bank 1 Sensor 2 downstream oxygen sensor sits after the catalytic converter and reports post catalyst oxygen content to the Powertrain Control Module (PCM) as a switching voltage, high for rich and low for lean. DTC P1138 sets when this downstream signal stays above about 0.45V, indicating the gas leaving the converter is rich or the sensor is biased high. This can reflect a genuine rich running condition that has overwhelmed the converter, or a downstream sensor or wiring fault. The driver may notice a fuel smell, poor economy, or no obvious symptom, with the MIL illuminated and possibly ...

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