Dishwasher not draining: is it the drain or the machine?

You can tell which side the problem is on by comparing what you see when the machine drains with what you see when you use the sink. The first three signs point to the drain, and the last three point to the machine.

To make this comparison easier, pour a bucket of water into the sink and then run the machine on a drain-only programme straight afterwards. If the sink swallows the bucket of water easily but water rises from the sink once the machine starts, the line cannot cope when the two flows join, and the narrowing starts after the trap.

The most common cause on the machine side is the filter at the bottom of the tub filling up with food scraps, glass fragments or bits of label. When the filter is full, water cannot reach the pump properly and sits at the bottom of the tub. The position of the filter and the way you remove it vary by model, so it is best to check your user manual, though the general order is similar for most machines.

If the filter is clean but the machine still does not drain and the sink runs freely, the drain pump is next. Hearing a hum from under the machine during draining without hearing the sound of water can mean the pump cannot turn or that something is stuck in it. No sound at all is a sign of an electrical problem or a pump failure.

The pump and the electronic control board are internal parts of the machine, and the machine's service technician should deal with them. A drain unclogging crew cannot help you in this case, so it is important to check the filter and the sink first to avoid an unnecessary expense.

If the machine stops draining halfway and shows a warning, make a note of it and tell the service technician when you call. Because the meaning of a warning changes by brand and model, comparing the code on the display with the manual helps you work out more quickly whether the fault is in the pump or in another part.

If the machine was recently installed or the space under the worktop was rearranged, check the drain hose. When the machine is pushed into place, the hose can fold at the back or get crushed under a cabinet foot. A folded hose narrows the passage of water, and the pump struggles to push the water out.

The second critical point for the hose is its height. Before it connects to the trap, the drain hose should be fixed so that it loops upwards under the worktop. This loop stops dirty water from the drain flowing back into the machine. If the hose lies on the floor, or connects to the trap below its own level, water from the sink seeps into the machine and foul-smelling water collects in the tub. Follow the measurement in your machine's manual for the height of the loop.

If the sink and the machine are connected to the same trap, and the trap or the wall pipe after it narrows with grease, the water the machine pushes out has nowhere to go. The pump runs and some of the water goes down, but the rest rises into the sink or stays in the machine. Some machines stop the programme halfway and show a warning in this situation. What needs to be done here is to clear the line rather than fiddle with the machine.

The side spigot of the trap, where the machine's hose is attached, can also be blocked on its own. If only this spigot is blocked, the sink drains freely while the machine cannot empty. On some trap models, the thin membrane inside this spigot is left uncut during installation, and a newly fitted trap can show the same symptom.

If you are going to take the trap apart yourself, put a bucket under it, loosen the machine's hose clamp and detach the hose, and undo the nuts by hand. After cleaning, make sure the seals are properly seated, tighten the nuts by hand again and test the machine with a short drain.

Once you have compared the symptoms, the decision usually makes itself.