iston Combi hot water & Central heating System Red Light K?

Two weeks ago we had a leaking joint on one of our radiators this was repaired by a plumber who also drained the system of air. Since then the Ariston Combi unit keeps on shutting down with the Red light showing. We have tried the system with only the hot water (Summer setting) on. The unit worked ok for one day then Unit began shutting down the Red light began showing fault again. We called out the plumber again but he said we needed an Ariston Technician. He came a few days later and diagnosed an electrical problem for which he needed a part which he did not have at that time. This all happened in our house in Spain & we were returning home on that Friday. We were able to ask a friend who would see that he was able to replace the part. Which he duly did and he said all was ok. my friend has since checked the system and the fault has reoccurred; The water pressure up around 2bar and the system usually operates @ 1.5bar, any ideas.