Code Definition

P1138 — Lack Of HO2S12 Switch - Sensor Indicates Rich

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

About P1138

P1138 is a manufacturer-specific powertrain diagnostic trouble code. 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 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

This code sets when the powertrain control module (PCM) detects that the downstream heated oxygen sensor (HO2S12) is not switching between rich and lean conditions as expected and remains stuck indicating a rich condition. The downstream sensor monitors catalytic converter efficiency, and a persistent rich signal may indicate sensor issues or exhaust-related problems.

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