Our heating system seems to be operational... specially given the fact that 2 out of the three zones work like a charm.

The zone that doesn't work uses a "Honeywell V8043 E 1012 motorized valve", and since the problem started, we've replaced the motor only... and it didn't work. Now, we've replaced the entire valve, and we still have the same issue. In addition, if we set the valve to ON (instead of AUTO), the heating will work, but it'll warm up too much as there's no governor to tell it to stop heating.

We've also switched thermostats from the zones that work to the non-working one, and no deal.

Can someone please give us some ideas of what to look into to better troubleshoot and determine the root cause for our problem?. as I'm writing this, it's making me believe that the wiring from the 2nd floor (problem zone) to the basement may be at fault... hope that's not the case. Anyway, any insight will be greatly appreciated.

Thanks,
Gus W.

You might have nailed the problem but before you jump to replacing the wires check the valve and wires out.
Check to see if there is a signal source/24 volts going to the valve motor.
Next check the end switch operation if the valve has a end switch.
Next tie the 2 wires for the end switch together (jump wiring) this is supposed to send a signal to the boiler to start up or start the pump. See what happens.

Report back with your findings.

A diagram for you.

http://customer.honeywell.com/techlit/pdf/95c-00000s/95C-10932.pdf

Thanks for all the info. We'll check and let you know how it all worked out.

Thank you again. After all, it was the wire from the valve to the thermostat in the 2nd floor that was the problem. The information you provided was very helpful, nonetheless, in narrowing down the problem.