Rohl C7912 Country Bath Cartridge Only for Thermostatic Valve A4910 A4913Bo A4917 and A4717 Cartridge only For Thermostatic Valve A4910A4913Bo A4915Bo And A4916Bo Remove With 17mm Or 11/16 Inch Open Jaw Wrench Please note: Test the installation before replacing the cartridge. On new installations, most cartridges have to be calibrated. This is the most common reason for replacement. On older installations, take the cartridge out and flush with high pressure. This occasionally returns an older cartridge to working condition.