DTC P1515: Commanded & Actual Throttle Position Mismatch: Description
WARNING: This page is about a different car, the 2002 Pontiac Firebird and 2002 Chevrolet Camaro. However, it is still accessible from the selected car via links, so may be relevant.
The commanded Throttle Position (TP), based on Accelerator Pedal Position (APP) and possibly other limiting factors, is compared to the actual throttle position. The Powertrain Control Module (PCM) monitors the commanded and actual throttle position. This DTC sets if the PCM detects the condition.