Toilet keeps running? The 5 causes, the dye test, and the $5 part that fixes most of them

The 5 causes ranked

(1) Worn flapper — ~60% of every case. The single biggest killer is chlorine tablets in the tank. Chlorine eats rubber. Flapper goes warped, brittle, or grimy, stops seating flat on the flush valve, water trickles past it into the bowl 24/7.

(2) Chain too short or too long. Too short — flapper never fully closes. Too long — chain pinches under the flapper when it drops, holding it open a crack. About a half-inch of slack at rest is the sweet spot.

(3) Fill valve (ballcock) stuck or set too high. When the water level is set above the top of the overflow tube, water just spills down the tube continuously, and the fill valve keeps running to chase it. The water line should sit about 1 inch below the top of the overflow tube (most tanks have a faint molded line — "WL").

(4) Flush valve seat corrosion. On older toilets (pre-2000), mineral buildup or pitting on the plastic/brass ring the flapper seals against. Run your finger around the seat. Smooth = fine. Gritty or pitted = no flapper will ever seal it. Fix: "fix-a-flush" repair ring (Korky makes one) or full tank-off flush valve replacement.

(5) Float misadjusted. On older ball-and-rod floats, the rod gets bent. On modern cup floats, the adjustment clip slips. Symptom is the same as #3 — water rides too high and spills the overflow tube.

The 60-second diagnostic

(a) Listen first. Stand still in the bathroom for 30 seconds with the door closed.

• Hissing/hum = fill valve actively running. Water is escaping somewhere and the valve is chasing it.
• Faint trickle into the bowl = flapper leak. Water is bypassing the flapper into the bowl.
• Both, on a cycle every few minutes = "phantom flush." Slow flapper leak drops the tank level until the float trips the fill valve, valve refills, repeats forever.

(b) The dye test. Take the tank lid off. Drop 5-10 drops of food coloring (or a dye tablet from the hardware store) into the tank. Do not flush. Wait 10-15 minutes. Check the bowl. Color in the bowl = flapper is leaking. This is the EPA's own recommended test and it's the fastest way to confirm cause #1 without taking anything apart.

(c) Lid off, watch it work. Flush once. Watch the flapper drop — does it seat flat? Does the chain go slack? Watch the water rise — does it stop at the WL line or keep going into the overflow tube? You've now seen the whole machine in 15 seconds.

The flapper replacement — 10 minutes, $5-$15

Universal (2-inch flapper): Fluidmaster 504, Korky 2010 / 100BP. Fits maybe 80% of toilets — generic American Standard, Glacier Bay, older Kohler, most rentals. $4-$8.

Brand-specific: Newer American Standard, Kohler, and TOTO use 3-inch flush valves for stronger flushes. A 2-inch universal will never seal — you need a 3-inch flapper or a brand-specific kit. Korky makes brand-stamped flappers for each.

Korky vs Fluidmaster: Both fine. Korky uses their "Chlorazone" red rubber that resists chlorine and chloramine — if your water is hard or chlorinated (most municipal), spend the extra $2 on Korky. Made in USA, 5-year warranty.

Install:

1. Shut off the supply valve at the wall. Flush. Sponge out the last inch.
2. Unhook the old flapper ears from the flush valve posts. Unclip the chain from the flush lever.
3. Slip the new flapper onto the posts. Clip the chain with about a half-inch of slack when the flapper is closed.
4. Turn the water back on. Let it fill. Flush twice to confirm seal.

The fill valve replacement — 20 minutes, $10

If the diagnostic points to fill valve (constant hum, water spilling into overflow tube, float won't shut off), one part fits virtually every toilet in North America: Fluidmaster 400A, ~$10 at Home Depot.

Install:

1. Shut off supply at the wall. Flush. Sponge out the tank.
2. Disconnect the supply line at the bottom of the old fill valve (have a small bucket — there's residual water).
3. Unscrew the lock nut underneath the tank. Lift the old fill valve out.
4. Adjust the new 400A to height — it should sit about 1 inch above the overflow tube.
5. Drop it in, tighten the lock nut hand-tight plus a quarter turn (over-tightening cracks the tank — I've seen it happen).
6. Reconnect supply line, turn water on, adjust the float clip so the waterline sits 1 inch below the overflow tube top.

When to call a pro

DIY this 95% of the time. Call a plumber when:

• The flush valve assembly itself is broken — the whole plastic tower the flapper sits on is cracked or the seat is corroded beyond a repair ring. Means removing the tank from the bowl, swapping the gasket, re-bolting. Doable, but 90 min and a 30% chance of needing a second hardware-store trip.
• Cracked tank — hairline at the bolt holes is common on tanks older than 25 years. Don't patch. Replace the tank or the whole toilet.
• Slab leak symptoms — toilet running plus unexplained warm spot on the floor, sound of running water with all fixtures off. That's not your toilet — that's a supply line under the slab. Call.

FAQ

Related guides

• Low water pressure whole house — when the problem isn't the toilet
• Sewer smell in house — wax ring + supply diagnostics
• Water heater banging popping — adjacent plumbing diagnostic

Editorial standards: Cited authorities include EPA WaterSense Fix a Leak Week (the 200 gal/day figure) and EPA WaterSense Leak Facts. Reviewed by Al, Building Doctor — IUOE Local 39 Stationary Engineer.