You should test the OVP module itself.

Remove it from the aircraft.

On the workbench, connect it to a well-regulated adjustable DC power
Supply capable of delivering 3A Plus to OVP Input and frame ground.
Use a good quality Digital Voltmeter across the supply to measure the
voltages described below:

Connect a 12V 1156 automotive turn-signal lamp between the OVP Output
and ground (two lamps in series if your aircraft is 28V).

Starting with the supply set to 12V (28V), the lamp(s) should be on.

Slowly turn up the voltage. The lamps should go off when you reach
15.5V (31V).
If at that point, you begin reducing the voltage, the lamps should stay
off even as you reduce the voltage down below 5V (10V). The OVP has
"Tripped".

You have to momentarily turn off, or disconnect the supply and start
with the voltage set below the cut-out voltage to get the lamps back
on. "Reset" the OVP.

Set the supply to about 14.5V (29V) with the lamp(s) on. Rap on the OVP
with a screwdriver handle to see if vibration will induce premature
tripping. If so, replace it.

If the cut-out occurs less than 15.5V (31V), you may wish to adjust
the OVP so that the trip point is 15.5V or slightly higher.

Good luck.