P1515 on GM vehicles means the engine control module (ECM) has detected a performance problem with the throttle actuator control (TAC) system, specifically that the actual throttle plate position does not match the commanded position within an expected margin. The ECM monitors the throttle actuator motor and the throttle position sensors, and when the correlation between commanded and actual position falls outside the calibrated range, it sets this code and typically forces the engine into reduced power mode.
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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