Code Definition

P1138 — O2 Sensor High Voltage During Decel fuel Cutoff

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

About P1138

P1138 is a manufacturer-specific powertrain diagnostic trouble code recorded for GM 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 signal or supply voltage higher than the expected operating range; 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 General Motors specific code monitors the upstream oxygen sensor during Deceleration Fuel Cutoff, the closed throttle coasting mode where the PCM stops injecting fuel to save economy and reduce emissions. With no fuel being delivered during Decel Fuel Cutoff the exhaust should be full of air, so the upstream Heated Oxygen Sensor should read low voltage near 100mV reflecting a lean condition. P1138 sets when the sensor instead reads high voltage during this mode, indicating it is biased rich, contaminated, or that fuel is reaching the cylinders when it should not. The driver usually has no...

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