GS300 & Is300
WARNING: This page is about a different car, the 2003 Lexus SC 430, 2003 Lexus LS 430, 2003 Lexus IS 300, 2003 Lexus GS 430, and 2003 Lexus GS 300. However, it is still accessible from the selected car via links, so may be relevant.
Disconnect accelerator pedal position sensor connector. See Fig 1. Measure resistance between terminals E2 and VC. If resistance is between 1200-3200 ohms, accelerator pedal position sensor is okay. If resistance is not as specified, replace accelerator pedal position sensor.