Hi,
The [https://items.marco.it/media//attach/booklet/UP6-E_00_US.pdf|manual] states : "The electronic pressure sensor, through the use of a microprocessor, controls the pump's speed to obtain the needed flow rate...".
Just wondering if the sensor is the problem. The sensor is not shown in the parts list (see p.11) but maybe if you trace the wires from the [https://marco-pumps.shop/marco-marco-spare-part-r6400039-electronic-controller-12-24v-for-up6-e-r6400039/|electronic control] (see part #23 in manual) you may find it.
I'll tag @teof as he knows a lot about Marco pumps. Allow a bit of time for a response due to time zone differences