Code Definition

P1138 — Lack of Downstream Heated Oxygen Sensor Switch - Sensor Indicates Rich - Bank 1

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

About P1138

P1138 is a manufacturer-specific powertrain diagnostic trouble code recorded for Ford 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

This DTC sets when the Powertrain Control Module (PCM) detects that the downstream Heated Oxygen Sensor (HO2S) on Bank 1 is not switching and continuously indicates a rich condition (high voltage) during closed loop operation. The downstream sensor sits after the catalytic converter and is used to monitor catalyst efficiency, normally holding a fairly steady voltage. A sustained high, rich biased signal suggests the sensor is stuck or contaminated, the signal circuit is shorted high, or a genuine rich exhaust condition is reaching the sensor. The driver may notice an illuminated check engine l...

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