Maybe there was a voltage issue with the FTP wiring.
I'd find a wiring diagram, and see where the wires from the FTP go to if the voltage is reading low, then ohm test each wire form end to end to see if there are any shorts in it..
From that post above, just going to post the whole post from the above link.
FUEL TANK PRESSURE SENSOR LOW VOLTAGE
PCM monitors fuel tank pressure sensor signal to detect
vacuum decay and excess vacuum during the enhanced EVAP diagnostic.
Fuel tank pressure sensor measures the difference between air pressure
(or vacuum) in the tank and the outside air pressure. PCM applies 5-
volt reference and ground to the sensor. Sensor will return a signal
voltage between 0.1-4.9 volts. If PCM detects a fuel tank pressure
sensor signal that is excessively low, DTC will set.
You likely won't notice any drivability problems.
Fuel tank pressure sensor signal is less than 0.1 volt.
Check for poor terminal connections at fuel tank pressure
Check for shorts.
Check for damaged harness.
Replace fuel tank pressure sensor.