Turn off the electrical supply to the Trane furnace at the main panel. Switch the designated circuit breaker to the "Off" position.
Remove the thermostat cover from its wall mount to access the wiring terminals. Pull the cover straight off from the wall bracket.
Locate the terminal marked “24 C” This is the terminal connection for the 24-volt common wire.
Strip half inch of insulation from the blue wire of the thermostat harness, using wire strippers.
Connect the blue wire to the "24 C" terminal. Tighten the terminal screw clockwise, using a flat-head screwdriver.
Reinstall the thermostat cover if all other work is complete.
Remove the electrical access panel from the lower rear of the Trane furnace. Lift the panel up slightly, then pull it out from the bottom.
Locate the thermostat control terminal bus. The bus is mounted vertically with four terminals marked "G," "W," "R" and "C" "C" is the designated terminal for the common wire.
Strip half inch of insulation from the blue wire from the thermostat, using wire strippers.
Connect the blue wire to the "C" terminal. Tighten the terminal screw with the screwdriver.
Replace the furnace cover when work is completed.