Let’s tackle the why should I care question first. A running toilet is often times loud and annoying. You expect the noise when you use the toilet, but it can become unbearable when the toilet runs and wakes you up in the middle of the night, or
runs during the scary scene of a Freddie Kruger movie! Also, lets not forget that water is not free. A running toilet can waste over $1000 per year.
There are 3 possible guilty offenders that will cause a toilet to run:
1) The Flapper
2) The Fill Valve
3) The Flush Lever or Handle
The best way to diagnose a running toilet is to remove the lid from the back of the toilet and observe. Different symptoms will point you to the offender more quickly.
If your toilet is constantly running, this usually means that water is leaking from the tank into the bowl. The toilet will be continually trying to fill up the tank (ie constantly running).
The flapper (sometimes referred to as the tank ball) is probably not forming a proper seal to stop the water in the tank flowing into the bowl. Check for a proper seal by running your fingers along the seal side edge. If you detect any ridges, bumps, tears, etc. you will need to change the flapper. Also make sure to check what the flapper seals against. Sometimes debris can get caught causing an imperfect seal. If the flapper checks out ok, then check the flush lever or handle. Sometimes the chain will get bound up and hold the flapper in the open position.
If those items are all working correctly then the fill valve may be out of adjustment. The fill valve is set to shut-off when the water reaches a predetermined height in the tank. If the adjustment is set too high, water will flow into the overflow tube and into the bowl. In this scenario the fill valve will never shut-off creating a constantly running toilet.
If your toilet only occasionally runs, sometimes called Phantom Flushing, most likely your flapper needs to be replaced.
When the flapper (tank ball) deteriorates to a point, it won’t seal well and can allow water to seep into the bowl. When enough water has left the bowl, it will activate the fill valve to fill the toilet tank back up. Run your fingers along the sealing edge of the flapper to check for deformities that may prevent a tight seal. Also check the rim of the flush valve. This is what the flapper seals against. Because it may have deformities as well.