The 651 tells you there's an out of tolerance voltage on the 5-volt reference 2 bus which provides a constant 5 volts to the following sensors:
Accelerator pedal position sensor 1
Throttle position sensors 1 & 2
Crankshaft position sensor
So all of those sensors will be affected by the loss of that reference voltage signal which can cause a variety of driveability issues. Somewhere on that bus a wire is either shorted to ground or to voltage or one of the sensors themselves is. Could be occuring intermittently or could be a hard fault.
If you are skillled using a DVOM, here ya go:
• Additional DTCs will set when disconnecting the components.
• A short to voltage on the signal circuit of certain components may cause this DTC to set.
Ignition OFF, disconnect the harness connector of all appropriate sensors for the applicable DTC.
Ignition ON, measure for 4.8-5.2 volts between one of the 5-volt reference circuits and ground.
⇒ If the voltage is less than the specified value, test for a short to ground on all the 5-volt reference circuits. If all circuits test normal, replace the control module.
⇒ If the voltage is greater than the specified value, test for a short to voltage on all the 5-volt reference circuits for each component associated with the appropriate 5-volt reference circuit. If all circuits test normal, replace the control module.
Connect each component associated with the 5-volt reference circuit, one at a time, while monitoring the voltage. The voltage should not change more than 1 volt.
⇒ If the voltage display increases more than the specified value when a component is connected, test the signal circuit for a short to voltage. If all circuits test normal, replace the control module.
⇒ If the voltage display decreases more than the specified value when a component is connected, replace the component.