Anyone know if there is supposed to be an o ring on it? I put an o ring on and got a low psi reading.

 

No o-ring, because it's a pipe thread. The o-ring you installed may not be allowing a good ground.

 

It's a three-wire sensor. It grounds through the harness.

 

I don't know what the internal circuitry is. One pair of wires might be for the light, and the other wire and the grounded threads may be for the gauge. I do know you're not supposed to use an O-ring on any pipe thread fitting. Pipe thread is tapered, and that reducing taper isi what makes the seal.

 

Man, this guy know his **** about pipe threads lol

I don't remember putting an o-ring on when I changed mine. It might not be a bad idea to put some plumbers tape on it though.