A toilet that refills every few minutes often signals a small adjustment rather than a major plumbing failure. Common causes are a flapper that does not seal, a chain with the wrong amount of slack, a float set too high, or a fill valve that is not shutting off cleanly.
Listen and observe
Remove the tank lid and watch the water after a flush. If water ripples into the overflow tube, the level is too high or the fill valve is not closing. If the tank fills and then slowly drops, the flapper or flush-valve seal is leaking. A few drops of food coloring in the tank can confirm a flapper leak; wait about fifteen minutes without flushing and check the bowl.
Check the chain and handle
The chain should lift the flapper fully and leave a little slack when the flapper closes. A tight chain can hold the seal open; a loose chain can tangle beneath it. Straighten it and adjust one or two links at a time. Make sure the handle returns to its resting position.
Inspect the flapper and water level
Turn off the shutoff valve and flush to lower the tank. Examine the rubber flapper for warping, mineral crust, or cracks. Wipe the sealing rim with a soft cloth. Replace brittle rubber with a part matching the valve size and hinge style. Restore the water and set the level below the overflow tube using the built-in adjustment. Do not force plastic parts.
Know when to stop
If the valve continues to run, or the shutoff valve will not close, replace the faulty valve or call a plumber. Shut off the main supply if a connection leaks and cannot be isolated. Never use drain opener in a toilet tank. After repair, flush several times and check the floor, supply line, tank bolts, and bowl base for leaks.