First, let me explain what a blower door safety switch is.
Your furnace is equipped with a plethora of safety devices - flame rollout sensors, flame sensors (not the same thing despite a similarity in name), limit switches, serviceman's switch - all designed to increase your safety. The blower door safety switch is a simple plunger that, when opened, cuts the power to your gas-fired furnace. Look to the right and you can see what they look like. Depending on the age of your furnace, you probably have one of these. The top one is for a Carrier, the bottom on is for a Ruud (I think).
If you have something 40 years old like a Lennox G8-100, forget it - the switch hasn't been around that long. However, some older models can be retrofitted. The switches are about $30-$50 - it's something to consider but the worst an inspector would be able to say about a lack of the switch is that the unit met the standards for safety at the time of construction but standards have changed and improved with the intervening years.
The purpose of the switch is to prevent your furnace from pulling toxic gases into your home. When the blower runs, it creates a powerful negative pressure zone (to test it, try removing the blower door cover with the unit running - it is much more difficult to do as the blower tries to suck the cover back on.) If you have a leak on the unit releasing carbon monoxide or especially if the furnace is located in the garage, the blower door needs to be tight to the unit to keep the gases from entering the home.
The Most Common Defect with a Blower Door Safety Switch?
Human error in the form of the service technician. The switches are reliable and usually require no maintenance to function properly. But the switch poses a problem for the HVAC tech who is working on the system. The blower compartment usually houses the motherboard for the furnace. To do any work on the board or the associated electronics in the compartment, the door needs to be removed. The system shuts down, precisely as intended.
Except the tech needs it running to perform his diagnostics. So, he tapes the switch closed and gets to work. When he finishes, he puts the cover back on, gives you the bill, and leaves. Forgotten is the piece of tape over the switch. The safety device is no longer functioning as intended due to predictable human error. I see this about two dozen times every year, making it relatively common.
This is not meant to bash the techs. Especially since I have called more agents than I can count to ask to be let back into a home where I've managed to leave behind a tool. At one point or another, everything single tool has been forgotten, so I get it.
It also strikes me as silly to write this up as a defect and have a HVAC technician return to fix it. A service call at $60-$100 to remove tape? Nope.
I remove the tape. I'm not supposed to - it's outside my standards of practice and puts the liability on me. I calculate it to be an acceptable risk and a nice deed for the day.
I also check to make sure that the furnace is working before I leave. If it isn't, that little piece of tape goes right back on and the whole thing gets written up.