P1145 on a GM vehicle means the powertrain control module has detected that the downstream heated oxygen sensor on bank 1, sensor 3, is not switching or crossing its bias voltage often enough during a calibrated test window. The computer is not reporting a dead sensor circuit; it is reporting that the sensor's cross counts, the number of rich-to-lean voltage transitions, fell below the expected threshold while the engine was running in closed loop.
Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.
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