P1138 on a GM vehicle means the powertrain control module (PCM) saw the upstream oxygen (O2) sensor signal stay at a high voltage during deceleration fuel cutoff, when the sensor should read lean. The PCM commands fuel shutoff on decel, expects the O2 sensor to drop to a low voltage, and flags this code when the signal remains high instead.
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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