Your Dishwasher Is Probably Fine: Three Plumbing Faults That Look Like a Dead Machine

Water sitting in the bottom of the dishwasher usually is not the dishwasher. Three things in the cabinet next to it cause most of these calls, and one of them catches out brand new installs.

The dishwasher finishes its cycle, you open it, and there is an inch of grey water in the bottom. The obvious conclusion is that the machine has failed, and the obvious next step is pricing a new one.

Hold off. In our experience most of these calls are plumbing, not appliance, and two of the three usual causes are free to fix. One of them catches out brand new installations, which is why it is first on this list.

Before anything else. Bail the standing water out with a cup and a towel, and pull the filter or strainer in the bottom of the tub and clean it. Half the machines we get called to have a filter nobody knew existed.

1. The garbage disposal knockout plug

This is the one that makes people feel foolish, and it should not, because it catches professionals too.

A new garbage disposal comes out of the box with a solid plug cast into the dishwasher inlet on the side. It is there so the disposal can be installed in a kitchen that has no dishwasher. If you do have a dishwasher, whoever fits the disposal has to knock that plug out with a screwdriver and a tap of a hammer, and then fish the loose plug out of the disposal body so it does not end up in the impeller.

Skip that step and the dishwasher drain hose is connected to a sealed wall. The machine pumps, nothing moves, and the water comes back.

How to tell: did you get a new disposal recently, or move into a house where one was fitted before you arrived? If the dishwasher has never drained properly since, this is almost certainly it.

2. No high loop, so the water siphons back

The dishwasher drain hose has to rise up to the underside of the countertop before it drops down to the disposal or the waste tee. That rise is called a high loop and it is what stops dirty sink water running backward down the hose into your clean dishes.

Without it, the hose runs level or downhill from the machine to the drain, and every time the sink drains a little of it siphons into the dishwasher. You get water in the bottom that did not come from the wash cycle at all, often with bits of food in it that were never on your plates.

How to tell: open the cabinet under the sink and look at the hose. If it runs straight across without rising, that is your answer. Fixing it is a matter of strapping the hose up under the counter, which is about a ten minute job.

Depending on how the drain is run, an air gap fitting on the countertop may be the right answer instead. Which one applies depends on the configuration, and we will tell you rather than guess.

3. The air gap is blocked

If you do have an air gap, that little chrome cylinder on the countertop behind the faucet, it can block. Food debris collects in it, and when it does the dishwasher cannot push water past it.

The tell is water bubbling up out of the air gap itself while the dishwasher drains. Twist the chrome cover off, lift the plastic cap underneath, and clean out what is in there. It is unpleasant and it takes two minutes.

When it is actually the drain line

If all three of those check out, the problem is further along. The dishwasher shares a branch with the kitchen sink, so anything blocking that branch backs the dishwasher up too.

The tell is easy: run the sink. If the sink is also slow, or if water comes up in the sink when the dishwasher drains, you have a blocked kitchen branch and not an appliance fault at all. That is grease, nine times out of ten, and it is covered in where kitchen grease actually goes. The fix is a cable at $175 to $450, or jetting if the same line has done this before.

See clogged kitchen sink repair for that side of it.

When it really is the machine

We are plumbers, not appliance engineers, and we will say so. If the drain path is clear all the way and the machine still will not empty, the drain pump or the control board is the next suspect and that is an appliance call.

The useful thing is that the checks above rule out the plumbing for free, so you are not paying an appliance engineer to tell you that a knockout plug was never removed.

If you want it looked at

We clear kitchen branches most days and we will tell you honestly which side of the line your problem is on. See dishwasher drain repair, drain cleaning, or the cost guide.

Why is there water in the bottom of my dishwasher?

Most often the drain hose cannot empty: either it is blocked, it has no high loop so the water siphons back, or on a recent disposal install the knockout plug was never removed. A small amount of clean water in the sump is normal, a full inch is not.

What is the knockout plug?

A new garbage disposal ships with a solid plug in the dishwasher inlet so it can be used without one. If whoever fitted the disposal did not knock it out, the dishwasher has nowhere to drain to. It is the single most common cause we see after a new disposal goes in.

What is a high loop and do I need one?

It is the drain hose routed up to the underside of the counter before it drops to the disposal or the waste tee. It stops sink water siphoning back into the dishwasher. Whether a high loop or an air gap is required depends on how the drain is run, and we will tell you which applies to yours.

How much does it cost to fix a dishwasher that will not drain?

If it is the branch line rather than the appliance, a single fixture cleared with a cable runs $175 to $450. The three checks in this article cost nothing and solve a good share of these calls.

Is standing water in the dishwasher a health problem?

Left long enough it will smell, and it is a food waste problem more than a plumbing one. Bail it out while you work out the cause rather than leaving it to sit.

Written by the team at Shoresteady Plumbing, Temperanceville, Virginia. Master plumbing licensed in Virginia and Maryland. Prices shown are typical ranges for the Eastern Shore, not a quote. Your written estimate is the only number that binds either of us.

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