DTC P1515: Control Module Throttle Actuator Position Performance: Description
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
The Engine Control Module (ECM) compares the actual throttle position to the commanded throttle position by comparing Throttle Position (TP) sensor readings to the estimate throttle position. P1515 will set if the estimated throttle position differs from a calibrated range from the actual throttle plate position (based on TP sensor values) for a certain length of time or if a throttle actuator circuit fault exists.