This is a Tekmar tn4 420 Boiler Reset Module. It is used but seems to work correctly. In the third picture you see it paired with the Tekmar 336 Zone Manager Module. The 336 is not included in this auction although it is available in another auction. I tested the 420 with the 336 and a 541 thermostat which I also will be listing soon. The 420 powered up, made the boiler contacts on a call for heat via the 541, displayed the boiler fire icon ,displayed the boiler supply and outdoor sensors and talked to the 541 which is wired to the 336. This control was changed out after the second or third (?) owner of a high end home wanted different thermostats. So it was working at the time. I have two of these from two former clients same story. I saved STUFF like this to help other customers in case they had a failure. Tekmar doesn't fail very often. This needs to be mounted in a Tekmar tn4 enclosure. If you don't already have one see my other auctions