Lennox G16Q3-50 Furnace with AC, Door Interlock Switch and Main Circuit Breaker
As I commented, my G16s have a hardwired power switch on the side. Or as others suggested, you should figure out which breaker controls the furnace circuit, and turn it off. (You should not have to shut off the main unless you are working inside the panel box itself)

But after looking over the wiring diagram for my G16, it does seem that the door switch does break the AC hot before it connects to anything else (including the thermostat transformer).  So if you kept out of the blower control box, you would be safe to work on the thermostat and related wiring.