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DTC codes / Mazda codes / P1128

P1128 · MazdaHO2S Bank 1 Sensor 1 Signals Swapped In Key On Engine Running Self Test

P1128 on Mazda vehicles means the powertrain control module (PCM) detected that the upstream heated oxygen sensor (HO2S) on bank 1 is reporting a signal that behaves like the downstream sensor during the key-on engine-running self-test. The computer expects the front sensor to switch rapidly and the rear sensor to hold a steadier voltage, but the signals appear swapped, so the PCM flags a rationality fault.

SystemPowertrain
Code typeMazda specific
SeverityMedium
Fix difficultyIntermediate
AreaFuel and air metering
Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage