You should see bubbles (like when your air control valves are open) from your heat return. That is a bulb type ozonator, so likely burned out after a couple of years. They can be a source of nuisance trips if the check valve has failed and the mazzei or return line is partially blocked from scale or such. If it isn't working anyway, disconnect it. If you repair/replace it, get a new ozone check valve too.