Code Definition

P1129 — Downstream Heated O2 Sensors Swapped

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

About P1129

P1129 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 P1129 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.

P1129 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 powertrain control module (PCM) sets this code when it detects that the downstream heated oxygen sensors (HO2S) are installed in the incorrect positions. The downstream sensors monitor the efficiency of the catalytic converters, and their signals should correspond to the correct bank. If the PCM detects that the sensors are swapped, it indicates improper installation or wiring issues.

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