If your toilet runs constantly and replacing the flapper did not solve it, the fill valve is the next likely culprit. The fill valve controls water flow into the tank and shuts off when the tank is full. When it fails, water flows continuously into the overflow tube. Replacing it is a 15-minute job that costs under $12.

How to Replace a Toilet Fill Valve

What You Need

Universal fill valve (Fluidmaster 400A or Korky 528 are the most common)
Adjustable wrench or pliers
Towel and small bucket
Sponge

Step-by-Step Replacement

Step 1: Shut Off and Drain

Turn off the water supply valve behind the toilet (turn clockwise)
Flush the toilet and hold the handle to drain the tank
Sponge out remaining water from the tank bottom — you want it as dry as possible

Step 2: Remove the Old Fill Valve

Disconnect the water supply line from the bottom of the fill valve (the threaded connection under the tank). Have a towel and bucket ready for residual water.
Inside the tank, disconnect the refill tube from the overflow tube
Under the tank, unscrew the large locking nut that holds the fill valve to the tank. Turn counterclockwise. You may need pliers or a wrench.
Lift the old fill valve straight out from inside the tank

Step 3: Adjust the New Fill Valve Height

The top of the fill valve should sit about 1 inch above the overflow tube. Most universal fill valves have a twist-to-adjust shank that telescopes to the correct height. Adjust it before installing.

Step 4: Install the New Fill Valve

Insert the new fill valve through the hole in the tank bottom
From underneath, hand-tighten the locking nut. Then tighten an additional half turn with a wrench — do not over-tighten
The rubber washer on the fill valve provides the seal. No plumber’s putty or tape is needed on this connection

Step 5: Connect the Refill Tube

Clip the small refill tube to the top of the overflow tube. This tube directs a small stream of water into the bowl during refilling, maintaining the water seal in the trapway. Trim the tube if it extends down into the overflow tube — it should just clip to the top edge.

Step 6: Reconnect the Water Supply

Thread the supply line connector back onto the fill valve tailpiece. Hand-tighten, then a quarter turn with a wrench.

Step 7: Test

Turn the supply valve on slowly
Let the tank fill completely
Check for leaks at both connections (under the tank and at the supply line)
Adjust the water level by turning the adjustment screw on the fill valve — the water should reach about 1 inch below the top of the overflow tube
Flush several times to verify the fill valve shuts off properly

Troubleshooting

Water still runs after replacement:

Check that the refill tube is not inserted down into the overflow tube, which can create a siphon effect
Verify the water level is set correctly — too high and water overflows
Ensure the flapper is seating properly — sometimes a new fill valve reveals a flapper issue that was masked

Leak at the base of the fill valve:

Tighten the locking nut slightly
Check that the rubber washer is seated properly and not pinched

Slow fill:

Clean the supply valve — sediment may be restricting flow
Check the fill valve screen (some models have a small filter at the bottom that catches debris)

Prevention

Fill valves typically last 5 to 7 years. Replace proactively if you notice the tank taking longer than usual to fill, the fill valve making unusual sounds, or intermittent running (sometimes called “ghost flushing” — where the toilet briefly runs every few minutes as the flapper slowly leaks).

Keep a spare fill valve and flapper in your tool storage. When the toilet starts acting up at midnight, you will be glad you have the parts on hand.