I'm trying to replace existing Honeywell controller for my furnace with Ecobee 3. The Honeywell controller does not use a C wire, however there are a black wire and blue wire in the wall behind the controller. From furnace board it seems to me the black wire is C wire so I use it in Ecobee wiring. Unfortunately Ecobee starts with wiring error message.
Attached are wiring on Ecobee, furnace board and furnace diagram. I also noticed there is "Fast-Stat Common Maker Thermostat Wire Extender" installed in the furnace.
From what I can see, the brown wire connects thermostat and furnace. It is split into white and red, and then fed into Fast-Stat 5000 transformer.