Taco zone valve failure

Toasty

I have a boiler with three taco zone valves.  One valve opens when I turn on the thermostat, but the boiler or circulator does not come on.  I took a power head off a working zone and put it on the non-working zone valve, and the problem was the same.  Could the problem be in the body itself?  Does something in the body complete the circuit to the boiler?  Can you just buy the parts for the body?  Thanks for any help you can give.