Before we talk about some easy ways to troubleshoot and repair your bath faucet, we want to give you our #1 rule: When in doubt, call a professional. Yes, you will be out $120+ for a service call, but that is much less than replacing a permanently damaged faucet.
Sometimes your faucet develops a small drip even though you have the handles turned off. One important thing to avoid is cranking the handles back further to make the leak stop. Before 1990 many manufacturers used compression valves. They had a series of washers and O-rings, and the handle could be cranked harder to make the valve seal more tightly.
Faucets today use ceramic disc valves. The stem (heart of the valve assembly) is a brass or plastic body with two pieces of ceramic inside. When the water is turned on or off, the ceramic discs move against each other, aligning for flow volume. If these are cranked down hard, they crack, and the ceramic disc stem is destroyed. It is not a catastrophic failure, but a ceramic disc stem could easily go for $25-$75+ depending on the manufacturer.
The average faucet will have a handle that needs to be removed with a screwdriver or an Allen wrench. Notable exceptions would be handles held on by rare earth magnets, and push on style. Both of these require pulling the handle off from another magnet or off of a plastic assembly holding the handle on. At this point, before progressing further, be sure you have your hot and cold water supplies shut off under the sink.
The next step will be some sort of bonnet nut – usually a metal one that threads onto the valve body and retains the ceramic stem (or cartridge). Once the bonnet nut is off, the stem can be removed. Underneath it you may find a rubber seat or spring, or neither. Typically, the cartridge may end up needing to be replaced anyway. The faucet lines we carry at Falk Supply have Limited Lifetime Warranties. This means the manufacturer will provide replacement parts (within reason) but will not pay for labor to install them. To receive the free parts the homeowner will need to call the vendor directly.
If you bought the faucet from us, we can provide a vendor phone number, a parts diagram, a proof of purchase, or even any troubleshooting guides the vendor offers. We, along with many of our plumbers, recommend buying your plumbing hardware from a supply house like Falk Supply. We are glad to lend our troubleshooting knowledge on items purchased from us. We can also provide you all of the aforementioned documentation to help you get in touch with the vendor and hopefully get no-charge warranty parts to fix things up.
Remember, when in doubt call a professional, and buy your faucets from a local wholesaler like Falk Plumbing Supply. Do not make any assumptions about how your faucet handles come off. We like helping with troublesome faucets. Get in touch with us and we will get you going the right direction.