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 Throttle Position (TP) sensor connector. Do not confuse TP sensor with accelerator pedal position sensor. Both sensors have 4-pin connectors and are mounted on throttle body See Fig 1. Measure resistance between terminals E2 and VC at TP sensor. Resistance should be 1200-3200 ohms at 68°F (20°C). If resistance is not 1200-3200 ohms, replace TP sensor.