A running toilet at night is typically caused by one of several common issues that can be fixed quickly with basic tools and no prior plumbing experience. The most frequent culprits are a faulty flapper valve that fails to seal properly, an improperly adjusted fill valve or float, or a worn-out diaphragm in the fill mechanism. These problems waste significant water and create disruptive noise, but solutions range from simple adjustments to inexpensive part replacements.

• Immediate quick fixes: Lifting the float arm or jiggling the handle can temporarily stop the water flow ^[3][7]
• Most common causes: Faulty flapper (60%+ of cases) or fill valve issues ^[4][5][10]
• Typical repair time: 5-20 minutes for most DIY fixes ^[4][8]
• Cost: Under $20 for replacement parts like flappers or fill valves ^[6]

Fixing a Running Toilet at Night

Diagnosing the Problem

Before attempting repairs, identify which component is malfunctioning by observing the toilet's behavior. A continuously running toilet usually indicates either water leaking from the tank into the bowl (flapper issue) or water continuously entering the tank (fill valve issue). Start by removing the tank lid and listening for hissing sounds or watching the water movement.

• Flapper test: Add a few drops of food coloring to the tank - if color appears in the bowl without flushing, the flapper is leaking ^[6]
• Float inspection: If water overflows into the overflow tube, the float needs adjustment ^[7][10]
• Handle check: A stuck handle may prevent the flapper from seating properly ^[10]
• Fill valve assessment: If water continuously enters the tank even when full, the fill valve diaphragm may be faulty ^[3][6]

Most sources agree that 80% of running toilet issues stem from either the flapper or fill valve components ^[4][5][9]. The flapper is a rubber seal that should completely cover the flush valve opening when the toilet isn't in use. When this seal fails, water continuously leaks from the tank to the bowl, triggering the fill valve to add more water.

Step-by-Step Repair Solutions

For flapper-related issues, begin by turning off the water supply valve behind the toilet and flushing to empty the tank. Inspect the flapper for visible cracks, warping, or mineral buildup that would prevent a proper seal. Clean the flapper seat (the opening it covers) with a scouring pad to remove any debris or mineral deposits ^[5]. If the flapper appears damaged or cleaning doesn't resolve the issue, replacement is necessary.

• Flapper replacement steps:
• Purchase a universal flapper (under $10) that matches your toilet model ^[8]
• Remove the old flapper by unhooking it from the lift chain and flush valve pegs
• Attach the new flapper, ensuring the chain has slight slack (about 1/2 inch) ^[9]
• Turn water back on and test for leaks

For fill valve issues, first attempt to adjust the water level by bending the float arm downward or turning the adjustment screw on newer models ^[7]. If the valve continues to run after adjustment, it likely needs replacement. Modern fill valves cost $15-20 and can be installed in under 10 minutes by following these steps:

• Fill valve replacement:
• Turn off water supply and flush to empty the tank
• Disconnect the water supply line from the bottom of the tank
• Unscrew the locknut securing the old fill valve from underneath the tank
• Insert the new fill valve, secure with the locknut, and reconnect the supply line
• Adjust the float to set water level about 1 inch below the overflow tube ^[8]

Several sources emphasize that these repairs require no special tools beyond an adjustable wrench and can be completed by beginners ^[4][8]. The most critical safety step is remembering to turn off the water supply before beginning any work to prevent flooding.