thank you in advance. I have been struggling to find a proper way to install my ecobee. It is again 'C wire' issue. My old thermostat uses 4 wires (red, yellow, green, white) and does not use c wire. I am attaching the wire diagram and the picture of wire connections.

I am suspecting the brown wire (which is not connected anywhere) is the common wire but not sure. I have a multimeter but am not sure where to connect to check 24 AC. That is, if I connect one line of multimeter to brown wire, then where should I connect the other line of multimeter to?

Sorry to ask a question which can be so obvious to someone. I have been searching threads but was not able to understand fully. enter image description here

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Add your C wire to the junction of the thin white wire and the fat blue wire

We can follow a rather easy logic here, since you have air conditioning. We know for a fact that the cable going off to the air conditioner compressor must connect to the Y and C wires in order to work.

Thus, since the yellow Y wire from the thermostat connects to the red wire from the A/C cable, we know that the other wire in that cable (namely, the white wire) must connect to C. As a result of this, we can pinpoint where to connect the C wire from your thermostat; namely, the junction of the thin white C wire to the compressor and the fat blue C wire from the furnace.

As a double-check, we can look at the wire your furnace's wiring diagram says is the return to the transformer, and lo and behold, it is indeed a blue wire. So, we know for certain that we have the correct place.

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