I'm making an assumption that at 15 years old, this is a q2 brass valve(red, white, blue and yellow buttons)?
Is this a well or "city" water? if it's city water, I'd look at the water meter and see if the needle is moving when nobody is running water. You could have something using water all the time, like a stuck toilet valve. If it's a well, your pump will keep kicking on and off all the time. Just to make sure it's not the system,bypass it and see if water is still running.
Are the leds on the bottom blinking? then you need a new battery. If not, then I'd remove the battery and unplug the unit for a couple of minutes. Put the battery back in, plug the unit in and reset all the settings. Hope this helps, Joe