TACO Pump or controller bad?

Novice homeowner here who doesn't plan to take on more than he can handle, but I would like to attempt to diagnose the issue before calling in a professional.

I have a 4 zone hot water system fired from an oil burner. One of the zones (zone 3) is for a radiant floor system on my ground level. I tested this system back in October to ensure it was working properly. Though when calling for heat from the thermostat, it did take a while for the circulator to kick in, it did eventually.

Fast forward to this weekend. I usually keep the radiant floor thermostat at 65 degrees then supplement with forced hot air when needed. I noticed this weekend that my ground floor temperature was at 60 degrees (what I keep the forced hot air at when not in use) which told me the radiant floor heating was not operational. Radiant floor thermostat indicated it was calling for heat. I checked the controller unit and the light for zone 3 was not lit, nor was the exit pipe from the circulator hot indicating the circulator was not (nor had been) running. I shut the thermostat off, then on again to see if the furnace would kick in and the controller would light up. Nothing. The other three zones work fine with regards to the controller light and the circulators kicking in, just not the zone that the radiant floor is on. I swapped out the thermostat this morning and same result.

The controller is a TACO SR504
The circulator pump is a TACO OOR-MSF1-IFC

So my questions are:
1. Is it possible for a single relay in a controller box to be bad?
2. Does a controller zone's light only light up if the circulator is running?
3. Is there a way to manually start a circulator to test it's functioning?
4. If a zone intermittently works, what is that a sign of?

Thank you!

Dennis