Before you replace the black bathroom faucet, make sure to remove the handle. If you see any leaks, you might need to replace the cartridge. Be sure to tighten the connections and replace it properly. If you don't have an access panel, you can always patch the hole and repaint the wall. The new cartridge should be installed in the same place as the old one. Once everything is installed correctly, you can turn on the handle and test the new faucet.