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P0138 — Post-Cat O2 Sensor High Voltage (Bank 1 Downstream)

P0138 is set when the downstream (post-catalyst) oxygen sensor on Bank 1 reports a voltage above the calibrated maximum (typically above 1.0 V) for an extended period. The downstream sensor normally sits at a relatively steady 0.6–0.8 V because the catalyst smooths the rich/lean oscillation it sees coming in. A reading stuck above 1.0 V is either a genuine very-rich condition reaching the catalyst, a shorted signal wire, or a failed sensor biased high.

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