The shower valve had lost all pressure. When the valve was turned to the ON position the pressure would be only for a second and then it would idle to nothing.
We proceeded to remove the trim as you can see in the picture. We then shut down the stops and replaced the cartridge.
After testing the valve again it all worked perfectly. Don’t you wish that every job was this easy! 
Granted, we did not have the cartridge with us and we had to return a second day but still we are happy that it turned out to be so straight forward.