Boom Pressure Gauge Not Reading Zero

Things to try/check;

Reset the zero point for the Boom Pressure Gauge.

Check Plumbing, There should not be an isolator on the boom pressure line, if there is one move sensor to boom side of isolator.

If none of this worked it could be a failed probe.

From the Hopper Gauge Owners Manual

Reset Boom Pressure Gauge Zero Point:
To reset boom pressure gauge zero point,

1) Confirm spray pump is ‘OFF’,

2) Pull display CB,

3) Press and hold DIM button,

4) Reset display CB ‘On’,

5) When ‘Cal’ is displayed, release ‘DIM’button.

This resets the boom pressure gauge zero point (and also displays Firmware Version, quantity and pressure Units and Aircraft Model settings).