The bootloader is coming up and then the Checking for PC means it is looking to see if a programming computer is connected. The fact that is stops there and times out means it is getting lost in the weeds. However getting up to Checking for PC is a sign that it may take a download.
My opinion is that the next step is to try a reload of the BIN file (panel operating system). Of course there could be a hardware issue.
Yes, it will take a Simplex tech to do the reload. I have the necessary equipment to try the reload if you want to ship the panel to me. Sorry, I can’t release any software to anyone. Plus I think it takes a key.
When I was working I never heard of a 4008 doing this. But, it is a computer and we all know they can be glitched.
I looked up the System Sensor Mass 24-1 devices and do not see a problem using them.