If you hear your toilet running, then you should probably check your toilet. This is normally caused by water continuously running out of the tank and into the bowl, and keeping your fill valve from shutting off.
You should check the Flapper or Tank Ball: The best place to start is by checking the flapper, tank ball or flush valve. These are the most common areas for problems to begin. When replacing or repairing the flush valve and its parts, make sure to turn the water to your toilet off. The shut-off valve is typically located behind the toilet.
First thing to do is to check the rim of the flush valve where the flapper or tank ball rests to seal the water in the tank. Is it smooth, or are there chips or rough spots? A rough, eroded or chipped flush valve can certainly cause your toilet to run and should be replaced if the rim has been compromised. If the rim of the flush valve appears to be nice and smooth but water is still passing by the flapper or tank ball and flowing into the bowl, then a bad flapper or tank ball is likely the cause of the problem.
common flapper with chain
A faulty flapper or tank ball (possibly cracked) can fill with water when the tank is emptying causing the flapper or tank ball to close prematurely. They can also become misshapen or deteriorated, preventing the necessary seal and allowing water to pass through to the bowl. There are many different types of flappers and tank balls, and some flush valves use a cylinder with a flat seal, so the method of repair will vary from toilet to toilet.
If your toilet has a flapper that attaches to a flush lever arm/rod by a chain, it's important to make sure the chain is correctly adjusted so it isn't too tight or too loose. If the chain is too tight, the flapper will allow water to run into the bowl because it can't seal itself sufficiently onto the rim of the flush valve. If the chain is too loose, the flapper won't be pulled up enough to stay open, thus not allowing the water to flush into the tank. Or, the chain can get lodged between the flapper and the rim of the flush valve causing the flapper to stay open and water to run into the bowl. The chain should have just a slight bit of slack in it, typically around a half-inch. If there is excess chain left once it has properly been adjusted, then cut the excess chain off with pliers or wire cutters
Checking the Flush Lever Assembly (handle) : Is the flush lever (also called a "trip lever") tightly secured to the wall of the tank? If not, try reaching into the tank and tightening the lock nut to be sure (hand tightening is ok and helps prevent the possibility of cracking your china/porcelain). A flush lever lock-nut usually tightens opposite from standard nuts, so reach in and turn the nut from right to left. Check and make sure all of the parts of the flush valve lever are intact and are not bending or coming apart from each other. If the flush lever appears to be in bad shape, you may just need to replace it with a new flush lever assembly. When you push on the trip lever does it stay stuck in the up position, preventing the flapper from dropping and closing correctly? If so, then you will need to replace your flush lever.
Then check the Fill Valve: When the flush lever is working correctly and the flush valve is sealed properly but water continues to fill the tank and exit into the bowl through the overflow tube then check the fill valve.
400A fill valve - the most common today
Toilet tanks have an overflow tube to prevent water from spilling over the top of the tank and onto the floor, but a fill valve is designed to shut off the water flow (when the toilet tank is full) to prevent the tank from over-filling (thus not allowing water to continuously exit the tank and into the bowl through the overflow tube). If your fill valve is not shutting off the water once the tank is sufficiently full, the problem may be with your toilet fill valve. This could be caused by dirt or debris in the valve, or it may be the fill valve needs to be repaired or replaced.
Next, inspect the fill valve to see if it can be repaired, you must first turn off the water. The shut-off valve can usually be found behind the toilet. If you have the type of fill valve that uses a float ball (which closes the valve when it rises) then first check and make sure the float ball doesn't have any cracks in it. If a float ball has water inside of it, it could be too heavy for the fill valve to lift in order to shut off the incoming water, causing the toilet to constantly run. It is also a good idea to make sure the float ball and rod aren't sticking, in which case you would most likely need to repair or replace the fill valve).
Different types: There are also fill valves that do not use a conventional float ball to shut off the valve. Instead, they are canister-type fill valves (such as the Fluidmaster #400A) which have a chamber that rises up the body of the fill valve to shut off the water supply. Over time, this type of float chamber can stick. If you are unable to stop it from sticking, then you should either repair or replace the valve.
OK - Check the internal seals of your flush valve to make sure they are in good shape. If they're ripped or disintegrating, you can try to locate replacement parts for your fill valve. Please note that not all fill valves are the same and are manufactured by different companies. Make sure to order repair parts for your specific valve.
Next: To check for dirt/debris, you can try removing the top cap from the fill valve (if your fill valve has a cap that can be removed) and the loose washers and parts from the inside of the valve. Look at the seals to confirm they are in good shape. Make sure to take notice of how you removed the parts so that you will be able to replace them later. Grab a plastic cup or similar vessel, place it directly over the center of the fill valve, and then slowly and carefully turn the water back on (low); the water will flow out through the center of the fill valve and should remove any debris from inside the valve. Next, turn the water back off so that you can re-install your fill valve parts and seals. Re-install the top assembly and turn the water back on. If the fill valve shuts off when the tank is full, then it would appear the problem has been fixed.
Finally, If you have replaced the flapper or tank ball (or determined the flapper or tank ball & flush lever are not the problem) and/or your attempt at repairing your fill valve was unsuccessful, you will most likely need to purchase a new fill valve. Pay careful attention to the type of fill valve that is used in your toilet, as again, some toilets require unique components that the manufacturer made specifically for their toilet models. It may be a good idea to contact the manufacturer of your toilet for their repair advice as well.
Call Evan if you can't figure it out.