Have a Trane hanging natural gas furnace with Mears thermostat (inside cover
Has M23-DP. 2-12 gauge red wires, 2-12 gauge black wires, and green 12 gauge wire to cover case. Have 3 volts on two wires with furnace running.
What are function of wires.
Thank You for any help. Robin

One of the black and one of the red wires will be your power supply reading 120V to ground (green), and 240V between Red-Black. The other red wire and black wire are the switch leg off the thermostat. When the stat is ON and the contacts inside closed, power will be sent to those wires. I don't know where you're reading 3V from, but it shouldn't be on any of these wires unless there is a 3V voltage drop from one red to the other. When posting voltages, you need to make clear where your leads are referencing or it's meaningless.