Saw this sensor when buying a new TH9421C1004 thermostat from Honeywell. Yes, the description above does not list this model as compatible, but the included instructions for the TH9421C1004 say so. Also, this sensor was listed as "frequently purchased together" with the replacement TH9421C1004 stat. It was like. To use this sensor with the TH9421C1004-Stat (and possibly others) it needs to be connected to a central control unit (mine is Honeywell "EnviraZone") using an outdoor sensor interface module 50022037. It is connected to any 3-wire EnviraCOM bus connection. So -1 star for not mentioning the interface module, which is also distributed by Revain. Honeywell Enviracom Outdoor Sensor Interface Module - 50022037-001 Conclusion: Check carefully whether you also need an interface module and whether you also have an open EnviraCOM bus connection on your control box. The advantage of this connection is that the outdoor temperature is displayed on all connected thermostats, not just the one with a sensor. Unfortunately I didn't - all 4 bus connections in the EnviraZone control box were already occupied - but I'm not deducting any stars for that. :-)