DTC codes / Mazda codes / P1128
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.
Guidance is general and written for Mazda vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.
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