Fixing a running toilet is one of the basic toilet repair jobs. Often the problem can be diagnosed and even repaired by an amateur plumber. There are a number of good how to articles on the Internet. In this summary article, I have put together the best suggestions from other articles in order to come up with the perfect flow sheet for diagnosing the cause of a running toilet.

1) The first step is inspection. Gently lift the lid of your toilet tank, put it aside and look inside

2) If the tank is full and the flapper is closed and water is running over the top of the overflow tube. Then the flapper is working properly, but the fill valve is working improperly.
a. Turn off the water at the toilet shut off valve
b. Check for a misalignment of the float arm. A misalignment of the float arm allows the water to rise too high and therefore to trickle over into the overflow pipe and continually run into the toilet.
i. Pull up on the float with your hand. If this action stops the flow then adjust the level of the float.
1. If the float is around the valve post, pinch the metal clip and slide the float down the wire.
2. If the float screws in, try unscrewing it a bit and moving it further down the float arm.
3. If there is a ball on an arm, try turning the small screws on top of the valve. Sometimes, you can also bend the arm further down.
4. Make sure the float ball isn’t touching anything else
5. Make sure the float ball isn’t leaking or filling with water.
6. No matter how you adjust the float ball, adjust it so the tank stops filling when the water is about an inch, (2.5cm), below the to;p of the overflow tube.

3) If the flapper won’t close: close the flapper if that stops the leak, consider the following:
a. Is the chain catching on something? dislodged or tangled
b. Is the flapper catching on the chain?
c. Is the float connected too low on a flapper chain (flapper chain float not the ball float) Try raising the float to see if this solves the problem.
d. If none of the above maneuvers solved the problem consider the following:
i. Is the flapper aligned with the opening?
ii. Is the flapper discolored on the bottom, (continuous leak)
iii. Is the flapper simply just old and stiff (and needs replacing)?
iv. If you answered yes to any of these questions, then the problem is an old torn or worn out flapper assembly
v. Replace the flapper

4) If you have a ball seal instead of a flapper, is the wire that lifts the ball straight and does it move freely?

5) Sometimes the leak is the results of sediment and water deposit build-up in the flush valve seat, this can prevent the rubber flapper from dropping into the seat properly, and leads to a leak.
a. Try thoroughly scrubbing and cleaning the valve seat and see if that stops the leak.

6) If the toilet stops filling and then starts again intermittently, you have a slow leak.
a. Place a dye tablet or a few drops of food coloring in the tank if after an hour or two without flushing, you see this dye in the bowl, you have a slow leak
i. The most common cause of slow leaks is a leaky flapper
1. Cleaning may work usually best just to replace the whole flapper
b. Replacing the flapper:
i. To perform the replacement, simply close the valve sticking out of the wall under your toilet tank,
ii. Flush the toilet
iii. Pop the old flapper off its hinges, disconnect it from the chain, and pop the new one into place
iv. Run a finger carefully around the rim where the flapper seats
v. Remove any uneven buildup of minerals that might cause a leak
vi. Use a sponge with bleach to clean the flapper seat.
vii. Open the valve all the way
viii. Try flushing a few times
ix. Trim and adjust the chain by trial and error

7) The toilet valve may be acting as a siphon and drawing water back into the fill tube.
a. Adjust the valve height or tube height up, or adjust the water level down.

8) If water is leaking through the fill valve, you need to consider replacing one or more of its parts or else the whole fill valve
a. If the rubber seals are worn, replace the rubber seals
b. One or more of the non-rubber components may break in the toilet’s water valve mechanism: -
i. Try fixing the broken part with epoxy
c. Replace the whole valve.