Code Definition

P1129 — Heated Oxygen Sensor Signals reversed

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

About P1129

P1129 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 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

This DTC is functionally similar to P1128 and sets when the Powertrain Control Module (PCM) detects that the behaviour of the upstream and downstream Heated Oxygen Sensors (HO2S) on a specific bank is inconsistent with their expected roles, suggesting that their signal or heater circuits may be reversed. During closed loop operation the upstream sensor should switch rapidly between rich and lean (roughly 0.1V to 0.9V), while the downstream sensor should hold a much steadier voltage as it monitors catalyst efficiency. When the PCM sees the downstream sensor switching like an upstream sensor, or...

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