Showing it's getting hot?

I have a 05 Buick Rendezvous , when the fans kick on the temperature gauge goes up to about 3/4 then when the fans kick off temperature gauge goes back down to about 1/4? Not sure what the problem is? Took it to have a diagnostic test done on it and it’s showing that the motor is not getting hot at all , it’s at 216, what it calls for, only other problem that showed up on the diagnostic test was the oil sensor,
also having problems getting the vehicle to start, doesn't want to start without giving it a little bit of fuel? Then it starts up? Parts replaced so far, A new Thermostat, A new temperature sensor, Fuel Injector, Idle Control Valve, Throttle Position Sensor. Please help !