Code Definition

P1128 — HO2S Bank 1 Sensor 1 Signals Swapped in Key On Engine Running Self Test

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

About P1128

P1128 is a manufacturer-specific powertrain diagnostic trouble code recorded for Mazda vehicles. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P1128 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.

P1128 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

During the key on engine running self test the Powertrain Control Module (PCM) manipulates fuel delivery and watches each upstream heated oxygen sensor (HO2S) respond to confirm each sensor is wired to the correct bank. DTC P1128 sets when the PCM determines the Bank 1 Sensor 1 signal responds as though it belongs to the opposite bank, indicating the upstream oxygen sensor connectors or wiring have been swapped between Bank 1 and Bank 2. This most often follows sensor replacement or engine work where the two upstream connectors were interchanged, but it can also be caused by crossed wiring or ...

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