Q: Reduced engine power 2004 Pontiac Grand Prix

Hi - the loss of power on your Grand Prix may indeed be the throttle position sensor or throttle body - as you suspect. Getting the problem codes which usually turns on the Check Engine light as well as forcing your engine into "limp mode" will help narrow down the failing component. An auto parts store will usually extract these for you for free on a "time available" basis. Otherwise, I would recommend a loss of power inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.