Custom faults

I have used the axis faults and spindle faults successfully, and I'm trying to find the best way to handle something similar.  I am trying to make a custom M Code to drive a hydraulic part off slide, and want it to fault if certain conditions are not met.  I can make a signal for the alarm, but am not sure what is the best way to tie it into linuxcnc like the axis faults are.  

I also want to do this for a number of other things on this machine.
Tool Change- I am using classic ladder but still need to work on fault handling. 
collet closer- It has a pressure switch input to indicate if it is closed. 
 

What kind of fault do you want? Just a message, abort, estop?
I don't think there is a really nice easy straight forward way to do this but there are ways to do it. mostly depends on what you want the fault to do.

I think at least stop any running program and show fault name. The way it works for spindle or drive faults would be perfect, but really anything that will stop the program and let me look into what has failed and why.

I'd make a python program that sends an abort command and an error message.
You could signal it with a HAL pin, or incorporate it in the screen directly, depending on what screen you use.

Look here at 'abort()' and 'error_msg()' commands.
linuxcnc.org/docs/2.9/html/config/python...command_code_methods

So abort would be useful if running the G code tool change remap. I'm not sure that would provide a solution when running g code and using classic ladder to do tool changes.