When a furnace blower door safety switch is taped.
Your HVAC system comes with a variety of safety devices, and they are all there to increase personal safety.
There is the flame roll-out sensor, the flame sensor (not the same thing), a limit switch and a serviceman's switch. And all these are for safety.
Another is the blower door safety switch.
It's a simple device, one that is pressed on the close the electrical circuit when the front panel is removed.
When the panel is in place the unit can run, and the blower will send air through the system.
That little switch is important!
It is intended to operate independently so that when the panel is removed to change the filter the blower mechanism pops out, opens the circuit, and the blower stops working.
If a finger was to get into that high-speed blower fan it could be snipped off!
You can see the filter to the right of the photo here.
I check filters during a home inspection, of course.
And from time to time when I remove the front panel I see that the switch has been taped down. And worse - here it was taped with electrically conductive aluminum tape! If somehow electricity touched that tape it would become live.
That means that the circuit is always closed, and the blower will keep running even when the panel is removed. Not smart. And not safe.
My recommendation: the blower door safety switch wants to be free! Let it go! Taping it down is not recommended, it is not good, and should not be done. Now, during this home inspection I did NOT removed the tape! I'm not touching it! We'll let that get taken care of later.