Vokera option boiler keeps triping out?

I have a "vokera option" boiler that won't stay on. it seems to trip out at the start up. It will light, burn a bit then trip out. If it will get past the start up it will sometimes run for some time but then trip out again later. Some times with the pressure light lit sometimes without. this has started after the heat exchanger was replaced. It also did this when it was first installed. A Vokera engineer came out and said it hadn't been set up correctly. I think he used the term "commissioned" is there anything that has to be set after the system has been drained? Any help gratefully received. Thanks.