Furnace start up. Found that there was not enough slack to pull new wires to replace. Installed add a wire to thermostat and terminal on furnace. Cycled AC , constant fan, and heat. All three modes operating normal at this time. Furnace start up. Cycled system. Checked incoming and outgoing gas pressures. Adjusted to maximize efficiency. Checked for gas leaks, none found. Checked wire connections, ok. Checked venting, drain lines, and operation. Checked temp rise, ok. System operating normal at this time. Job complete.