I am far from a JD expert, but since you have not gotten any replies to your question, let me offer this. I don’t have a 310G technical manual, but the 310SG manual says a F421 is ”Throttle Input Voltage Below Lower Calibration Limit”. There are various calibration and adjustment procedures listed based on serial number of tractor. These procedures are about 4 pages and involve the use of a multi-meter and other tools to access & adjust the sensor.
Perhaps you could visit a JD dealer with your 310G serial number and get the correct procedure for your unit.
I hope this helps.