Do you have a toilet that will start running and then suddenly stop for no reason? You need to replace the fill valve and/or the flapper. I’ll bet it’s the latter.
Your toilet is running because either water is running in the toilet or water is running out. Take the cover off of the tank and look at the overflow tube (typically a white tube about the diameter of a quarter). If water is running down this tube, your toilet is overfilling. The cause is either the fill valve or the float connected to the valve.
If the overflow tube looks OK, the cause is either the flapper or the lift chain. I would change both of these parts because when one goes, the other isn’t far behind. Changing these items is easy and inexpensive.
Many things can go wrong inside a toilet (besides the obvious), but changing these two parts will fix 90 percent of toilet problems.
First, turn off the water supply valve under the toilet. Next, flush the toilet; this will empty the tank. Unscrew the coupling that connects the water supply tube to the bottom of the toilet, and slowly pull out the tube. Now, unscrew the nut that holds the fill valve on and remove the valve. Have some rags and a small bucket ready to catch any excess water that didn’t drain from the tank (this water is clean).
Slip the rubber washers onto the new fill valve according to the manufacturer’s instructions, and insert the valve into the hole in the bottom of the tank. Screw the nut onto the valve to secure it in place and re-attach the water supply line. Hand-tighten only. Adjust the height of the valve, again per the manufacturer’s instructions, and connect the fill tube to the overflow tube.
To replace the flapper, disconnect the pull chain from the flapper, then pinch the tabs on the back of the flapper. It will lift right out. Install the new flapper by inserting the rear tabs into the retainer and connecting the pull chain.