I have a Buderus boiler. Two zones, Taco valves. Boiler isn't firing. I've replaced the transformer behind the switch. When I take a multi-meter to either the zone valve or the R and W leads in the boiler, the boiler fires. I have replaced the batteries in both thermostats. I also replaced ONE of the zone valves. I don't think I've screwed up the wiring, but who knows. I believe the white wires for the thermostats are going to terminal 1 on the valves. Any suggestions?

If your boiler fires when you bypass the thermostat then the thermostat is bad, be it the wiring or contacts. That's where I would start by eliminating the thermostat. Seems like you have already done this or am I misreading that you got the furnace to fire?

If Not then do THIS

How to Test the Thermostat on a Gas Furnace | Home Guides | SF Gate