Turn off the water supply to the toilet
Remove the tank lid
Check the flapper for wear, warping, or debris
Clean or replace the flapper if needed
Check the flapper chain for slack or tangling
Adjust the chain so it has slight slack
Inspect the fill valve for damage or malfunction
Adjust the fill valve water level if it is set too high
Replace the fill valve if it continues running
Check the float for proper movement
Adjust or replace the float if it is stuck or damaged
Inspect the overflow tube for cracks or incorrect water level
Make sure the tank water level stays below the overflow tube
Tighten any loose tank components
Replace worn tank-to-bowl or flush valve parts
Turn the water supply back on and test the toilet
Repeat adjustments if the toilet still runs