Code Definition

P1139 — Brake Booster Vacuum Sensor (Low Vacuum), or O2 Sensor Insufficient Switching (Bank 1 Sensor 2)

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

About P1139

P1139 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 P1139 is stored, the module has detected a signal or supply voltage lower than the expected operating range; 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.

P1139 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 has two platform dependent definitions, a Brake Booster Vacuum Sensor reporting low vacuum, or insufficient switching from the Bank 1 Sensor 2 downstream oxygen sensor. On vehicles with a brake booster vacuum sensor, the PCM uses that sensor to confirm adequate vacuum is available for power brake assist and to manage the engine when vacuum is low, and P1139 sets when the measured booster vacuum is lower than expected. On platforms using the oxygen sensor definition, the code reports that the post catalyst Bank 1 Sensor 2 is switching more than a healthy rear s...

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