What is a dishwasher air gap?
Updated August 2026
A dishwasher air gap is the small chrome cylinder on the sink deck that keeps dirty sink water from being siphoned back into the dishwasher. Inside it, the dishwasher drain hose ends in open air above the flood level of the sink, and a second, larger hose carries the water down to the disposal or a drain tee. Because the two lines never physically touch, no backup in the sink can reach clean dishes.
Where it is
Check the sink deck or the counter just behind the faucet for a chrome or brushed-nickel cap about 2 inches tall with slots around the top. The decorative cover lifts straight off, and the plastic cap underneath unthreads by hand. From below, the same fitting shows two hoses: a 5/8-inch line arriving from the dishwasher at the smaller inlet and a 7/8-inch line leaving the larger outlet toward the disposal. Kitchens without one usually rely on a high loop instead, with the drain hose strapped up tight under the countertop.
What it does
It is a code device with a single function and almost no way to wear out. What people call an air gap failure is water burping through the slots and spreading across the counter during the drain cycle, and that is a clog downstream, not a fault in the fitting. Two causes account for most of it. The knockout plug inside the disposal inlet was never punched out at install, so the dishwasher has nowhere to send water. Or food sludge has narrowed the 7/8-inch hose between the gap and the disposal. Both are 10-minute fixes with a bucket and a screwdriver. The plastic body itself only needs attention when a cap cracks or the chrome cover goes missing.
Maintenance
Clear it once a year, or the first time it spits. Pull the cover and cap, run a bottle brush through the inner tube, then loosen the lower hose clamp and flush the 7/8-inch hose into a bucket. Run the disposal for 10 seconds before starting any dishwasher cycle so the branch stays open. While you are down there, rinse the dishwasher filter, a monthly job in most kitchens, and descale the machine every 1–3 months if your water is hard. That routine is most of what carries a dishwasher to 9–12 years.
What it costs to replace
A replacement dishwasher air gap runs $10–20 for a new air gap body and cap, or $100–200 if a plumber handles it on a minimum service call.
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Frequently asked questions
Why is water coming out of my air gap?
Something downstream is blocked, so the drain water backs up and takes the only exit it has. Go in order: run the disposal and try again, then check that the knockout plug was removed from the disposal inlet if the dishwasher is newly installed. Next, pull the lower 7/8-inch hose and flush it, since that short run collects grease and grain. Last, clean the dishwasher filter. Very few of these calls end at a bad air gap.
Is a dishwasher air gap required by code?
It depends where you live. California requires one, and several other jurisdictions and many local amendments do too. Elsewhere, inspectors accept a high loop, meaning the drain hose is fastened to the underside of the counter above the flood rim before it drops to the disposal. Call the local building department for a straight answer, and expect a home inspector to flag a missing gap in a market that requires one.
Can I remove the air gap and cap the hole?
Only where code permits a high loop, and only if you install the loop properly. The vacated sink hole is typically 1-1/4 to 1-1/2 inches, which a soap dispenser at $15–35 or a chrome hole plug at $10–20 covers cleanly. The tidiest time to make the change is during a dishwasher swap, since most machines get replaced around year 10 and the drain line is already loose.