(99 GS300)
I have searched for multiple hours but can't find anything that will help me.
After plugging in OBD scanner, I got the codes listed above.
Which are....
-P1121: Acc. Position Sensor Range Performance Problem
-P0715: Turbine/Input Shaft Speed Circuit
-P1120: Acc. Position Sensor Circuit
How do I go about solving this problem,
and where should I start.
PLEASE!!! I NEED THE HELP.

 

Is the car running ok?

There once was a time when google was new and wasn't so good at predicting things where a search for something was fairly easy if you typed in a few words or characters based on logic. Example "Lexus 04 GS 300, code P1141". Now based on your habits it may think that means screwdrivers at Sears or a wrist watch. That's great if you are looking for a wrist watch but when trying to fix your car it's as if the search engine is being run by your ex or something.

The factory service manual is a handy thing. Me? I'm still trying to figure out the Toyota logic of how, and where they put information in it after being brain washed the Honda logic for 8 years. But so far what little time I've spent with it has been very helpful.

 

The P1121 and P1120 are "APP" (accelerator pedal position) codes. Not ACC btw. This has to to with the sensors in the throttle body. The APP is the one on the back facing the firewall that the cable connects to. I usually start by removing the throttle for a good cleaning then reinstall and clear codes/reset the computer and seeing if that does anything. The P0715 is a transmission code. Usually not a good one to have and calls for a new, used, or rebuilt transmission. But you can go to the front most speed sensor on the transmission and test it. This is what reads the input shaft speed. Check for power through the circuit, look for damaged wires or connectors. If you have a DMM check ohms of the sensor, pull the sensor out, look for metal junks on the magnet etc.