it could be possible the controller 5v system is no longer putting out enough voltage. Even though it is called a 5v system it actually just above 3.75 volts. This is enough for the throttle to send a signal to the controller to go full speed. If this voltage has dropped below 3.75 volts then the throttle will not be able to return enough volatage to the controller to tell it to go full speed and also will prevent a good connection via USB or Bluetooth
The way to check this is near the the wheel is a loose cable that does not have a plug on it and is commonly sealed from water with a blob of glue. Inside this cable are 4 wires. They are red, black, blue and yellow. What you need to do is open the end of this cable to reveal the wires. Then with the bike on (with no throttle applied) use a multimeter to measure the voltage between the red and black wires. Be very careful not to short out any of these wires as it will blow the controller immediately. If the measured voltage is below 3.75 volts then you need a new controller.
Here is a link to an older wiring diagram you can see the red and black wires.
https://www.goldenmotor.ca/FAQ/questions.php?questionid=99
This diagram shows the MP3 wire harness but all wire colours are still the same. you can see in the link the bottom drawing show the cable with the 4 wires inside by the wheel.
Gary
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