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P1178 on BMW vehicles means the engine computer has detected that the upstream oxygen sensor on bank 1 is switching too slowly between rich and lean exhaust conditions. The computer monitors the sensor's voltage response time and flags this code when the signal fails to cross the rich-to-lean threshold quickly enough during closed-loop fuel control.
Guidance is general and written for BMW vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.
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