67 twin turbo dart

370 so the sensor is good
Put a small piece of wire into the sensor plug holes check if the gauge goes to max .
Now if it does & you say if you short the 5V side to ground it goes to max then you have an open circuit in your return ground wire .What you can do is connect a long piece of wire to 1 leg of the multimeter ,connect the other multimeter lead to the ground return wire at the dash end to find your open circuit