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DTC codes / Ford codes / P1131
P1131 on a Ford means the powertrain control module (PCM) has detected that the upstream heated oxygen sensor (HO2S) on bank 1 is not switching between rich and lean as it should, and the sensor signal is stuck indicating a lean condition. The computer is seeing a lean exhaust reading from the pre-catalyst sensor on the bank that contains cylinder number one, and it has set this code because the sensor is not responding to fuel corrections as expected.
Guidance is general and written for Ford vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.
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