The filter gauge will read pressure anytime water is moving through the filter, whether or not a cartridge is in. If it’s stuck that way with the system off, you probably have a broken gauge. When you do put the cartridge in, the pressure should increase slightly, that becomes your baseline pressure. When it increases more than that (5-8ppm), it’s time to clean the cartridge. When you can’t get the cartridge clean anymore, or it has any rips, tears or damage, it’s time to replace the cartridge.