I have a 3 zone Lennox boiler system. One zone not working. I get hot water to two of the zones both supply and return. Non-working zone has hot water to the supply cold on the return. I first thought the circulator was out but it has current from the Aquastat, plus the other two zones have hot on both return and supply sides. When I touch the three Synchron, the two zones that have hot water are warm to the touch, non working is cold. I placed lever to manuel-no change.
Does this sound like the synchron is out, if so how do I replace?

Thanks