thanks for quick response I appreciate it.

I have tested the function of the OVP and it checks out ok (but could be intermittent I agree). I definitely dont have a ground signal from the PBU to the pressure switch although the wiring has continuity. Am in the process of examining the innerds of the PBU looking for a dry joint but none found so far.

Not sure how the OVP would stop the PBu grounding when PBU performing its other functions (valve control etc) ok - or is there something else I'm missing?

Keith