I put the sensor in and its still at the first mark when I'm driving but when I turned it off and turned the key to check the temp its almost at the normal temp

That's with the engine off. But driving still at first mark

Should I replace the thermostat?

Like cruiser said, weak/no heater if the stat was sticking open. Still sounds like something electrical.

Yeah ill have to check the connection. I just don't understand why when im driving it status cold but when its off it will show that its at normal temp

Electrical/grounds can be frustating and difficult to isolate......keep chasing.