Quick answer: A leaking dishwasher air gap is almost always caused by a blocked drain path — either the air gap cap itself is clogged, or the hose between the air gap and the garbage disposal/drain is blocked. Clean the air gap cap first (5-minute DIY job) — this fixes the majority of cases.
What Is a Dishwasher Air Gap?
If you've noticed a small chrome or plastic cap sitting on top of your kitchen counter, usually right next to the sink or built into the countertop — that's your dishwasher air gap. Many homeowners have no idea what it is or why it's there.
What it does: An air gap is a backflow prevention device. It creates a physical break — a literal gap of air — between your dishwasher's drain hose and your household plumbing. This prevents contaminated drain water from ever siphoning back into the dishwasher, which could contaminate clean dishes or drinking water. It works by routing the drain hose up through the air gap body before going down into the garbage disposal or sink drain.
The air gap sits on the counter and connects two hoses under the sink: a small hose from the dishwasher coming in on one side, and a larger hose going out to the garbage disposal or drain tailpiece on the other side.
When everything is working correctly, water from the dishwasher drain cycle flows up into the air gap body and then back down into the disposal/drain — you'd never know it was there. When something is blocked, water has nowhere to go and exits through the top of the air gap (which is vented to the atmosphere), spilling onto your counter. This overflow is intentional — it's the air gap alerting you to a blockage before a bigger plumbing backup happens.
Is an air gap required? Requirements vary. Many Ontario municipalities require an air gap on new dishwasher installations. Some codes accept a "high loop" alternative — where the drain hose loops up to the underside of the counter before connecting to the drain. If your home has an air gap, keep it — it's protecting your water supply.
1. Clogged Air Gap Cap DIY Fix — 5 Minutes
The most common reason an air gap leaks is the simplest one: the air gap cap itself is clogged. The cap has small openings that can collect mineral deposits, grease, and food debris over time. When these openings are partially or fully blocked, water can't flow through properly and backs up out the top.
Symptoms: Water drips or spurts from the top of the air gap cap during or just after the dishwasher's drain cycle (usually in the first 2 minutes of a cycle, or at cycle end). The water goes directly onto the counter or into a drip cup built into the counter fitting.
Fix: Clean the air gap cap — see the step-by-step instructions in the How to Clean Your Air Gap section below. This is a completely DIY repair that requires no tools and takes about 5 minutes.
2. Blocked Drain Hose After the Air Gap DIY Fix
Even if the air gap cap is clean, water can overflow if the hose running from the air gap outlet to the garbage disposal (or sink drain) is blocked or kinked. This larger hose (typically 7/8" diameter) runs under the sink and is the downstream leg of the air gap system. Grease and food buildup can accumulate inside this hose over years of use.
Symptoms: The air gap overflows even after you've cleaned the cap thoroughly. The blockage is confirmed when you disconnect the outlet hose and find water fills the air gap body but flows freely through the inlet side.
How to clear it:
- Put a small container or towels under the sink — there will be water
- Disconnect the hose from the air gap outlet (under the sink) by loosening the hose clamp
- Disconnect the other end from the garbage disposal or drain tailpiece
- Flush the hose with hot water to check for blockage
- If blocked, use a long bottle brush or flexible plumbing brush to clear the interior
- Reinstall the hose with the clamps firmly tightened — loose connections cause leaks under the sink
Also check that the hose isn't kinked or sharply bent where it goes behind the garbage disposal. Kinks dramatically reduce flow and cause the same overflow symptoms.
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3. Garbage Disposal Knockout Plug Not Removed DIY Fix
This cause is specific to homes where a garbage disposal was recently installed or replaced. Garbage disposals come from the factory with a plastic knockout plug sealing the dishwasher inlet port. When a plumber installs the disposal, they must knock out this plug with a screwdriver and hammer to allow dishwasher drain water to enter the disposal. If they forget (it happens more often than you'd expect), the dishwasher drain has nowhere to go and every drain cycle causes the air gap to overflow.
How to check:
- Under the sink, locate the nub on the side of the garbage disposal where the dishwasher drain hose connects
- Disconnect the drain hose from this nub
- Shine a flashlight into the nub opening — if you can see a solid plastic plug blocking the hole, it was never knocked out
- Use a flat screwdriver and a hammer to knock the plug into the disposal body (hold the screwdriver against the plug and strike it firmly)
- Reach into the disposal (with it unplugged!) and remove the plug from the disposal chamber
- Reconnect the drain hose
This is an overlooked but common installation error. If your air gap started overflowing immediately after a new garbage disposal was installed, this is almost certainly the cause.
4. Broken or Cracked Air Gap Cap DIY Fix
The air gap cap itself can crack or break over time — especially cheap plastic caps that are cleaned with harsh chemicals or accidentally hit during counter cleaning. A broken cap allows water to spill out in larger amounts than a clogged cap would, and the water flow may continue even when the drain hose path is completely clear.
How to identify: Remove the cap and inspect it for cracks, warping, or missing pieces. If the cap is intact but discolored and mineral-stained, cleaning may restore it. If it's cracked or broken, replace it.
Replacement: Air gap caps and complete air gap kits are available at most hardware stores (Home Depot, Canadian Tire, RONA) for $15–$50 depending on material (plastic, chrome, brushed nickel). The cap simply twists off the body — the body itself rarely needs replacement unless the hose fittings beneath it are cracked.
How to Clean Your Dishwasher Air Gap (Step-by-Step)
Cleaning the air gap is the first thing to try when it starts leaking. This takes about 5 minutes and requires no tools.
What you'll need
- A small bowl or towel (to catch drips)
- A toothpick, thin wire, or narrow bottle brush
- Warm water
- Locate the air gap — it's the chrome or plastic cap on your counter near the sink, usually 1–2 inches in diameter
- Remove the decorative outer cap — twist it counterclockwise, or some models just pull straight up. You may see a set screw on older models
- Lift off the inner plastic cap — this reveals the air gap body with two small openings (the inlet and outlet channels)
- Clear both openings — use a toothpick or thin wire to poke out any debris. Look for buildup of grease, minerals, or food particles
- Flush with water — pour a cup of warm water through each opening to confirm flow is clear
- Clean the cap — soak the inner cap in warm soapy water for a few minutes and scrub with an old toothbrush
- Reassemble — replace the inner cap, then the outer decorative cap
- Test — run the dishwasher drain cycle and watch the air gap. No overflow means success
If the air gap still overflows after cleaning, the blockage is in the hose going from the air gap to the drain/disposal (see Cause 2 above).
When to Call a Professional
The vast majority of air gap leaks are DIY-fixable. Call a professional if:
- You've cleaned the air gap and cleared the drain hose but the overflow continues — this may indicate a deeper drainage issue, a kinked drain hose inside the wall, or a problem with the dishwasher's drain pump
- The air gap body (not just the cap) is cracked or the counter fitting is damaged — replacing the full air gap body requires some under-sink plumbing work
- There's a strong odor coming from the overflow water suggesting sewage backup — this is a plumbing emergency, not just a dishwasher issue
- You're not comfortable working under the sink with hose connections and hose clamps
Nick's Appliance Repair can diagnose drain-related dishwasher issues and determine whether the problem is the air gap, the drain pump, the drain hose, or the household plumbing. A $65 diagnostic applies toward any repair. Call (437) 747-6737 for same-day service across Toronto and the GTA.
Related issues: If your dishwasher has a drain problem and you don't have an air gap, see our guide on dishwasher leaking from the bottom of the door — drain backflow can also cause door leaks.