I have a Laars Mascot II combi boiler. It's not working for the moment because of what I suspect to be low water pressure coming from the city. The alert code on the boiler is "3.5" at startup then it moves to "98 1" (no decimal point between the 8 and the 1).
If you measure the city pressure and it is less than 20 psi this may be the problem.
I believe the mascot has a high head loss exchanger. If you are pumping out of it instead of into it I think that is likely your problem.