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P1137 on a GM vehicle means the downstream oxygen sensor on Bank 1 (Sensor 2, after the catalytic converter) is reporting a lower-than-expected voltage while the engine is in power enrichment, which is the extra-fuel mode used during hard acceleration or heavy load. The powertrain control module expects the rear sensor to switch or hold a richer voltage during this condition, and it flagged the circuit because the signal stayed lean or lazy.
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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