Bad Manifold Pressure readings

I have a Model 7 Kitfox that is new to me. It has a normally aspirated Rotax 912 ULS for a engine.

It has an electric manifold pressure gauge that reads about 23" on the ground with the engine shut down. In flight the gauge reads around 20" with full throttle at 1500'. The gauge does read less with less than full throttle. Question is it likely the sending unit or the gauge that is at fault?
The gauge face has a Vans logo on it. (if that's a clue)

A picture of the instrument would help or the brand & model number.

It could be either the instrument, sender or wiring.