Domestic dishwashers are practically standard equipment these days. David Herres outlines procedures for diagnosing and repairing them.

Domestic dishwashers differ from the commercial units installed in large restaurants and institutional kitchens.

In the latter the water is super heated, typically by externally supplied steam circulated through a heat exchanger. The idea is that very hot soapy wash water followed by a very hot rinse ensures consistent cleaning of dishes and glasses, without the need for inspection.

A conveyer carries dishes past top and bottom wash and rinse spray heads. Workers load and unload dishes at the front and rear ends of these big machines.

The domestic dishwasher is much more compact. Hot water may be supplied through a screw-on connection to the kitchen sink hot tap, with a discharge pipe placed in the sink. Otherwise the machine is plumbed into the hot water system and the drain.

A dishwasher mounted on swivelling casters can be rolled about the kitchen and parked out of the way when not in use. Because it is front loading, the top can be used for another purpose. Some home-owners place a large cutting board on it.

The residential model does not feed dishes through in a continuous flow like the large commercial units, so it operates on a timed cycle for cleaning, rinsing, and drying by an electric element and air blower.

When it comes to repairs, if there is no sound and no power light the repair may be simple. Check to see if the breaker in the entrance panel or load centre is tripped. If it is, reset the breaker and see if the dishwasher performs normally. If the circuit breaker trips immediately, there is a short circuit in the supply wiring or internally.

Caution is needed to ensure that the outer cabinet is not energised. Check it with a neon test light or a multimeter.

Where water and electricity come together in an appliance, especially if it is not new, there is potential for shock to the technician and the end user. For this reason, it’s a good idea to consult a plumber.

Such an appliance should be connected only to a ground-fault circuit interrupter. The receptacle (if it is cord and plug-connected) should be checked with a circuit analyser to make sure the wiring is correct and the equipment ground has continuity back to the service and system ground.

If the circuit breaker holds, but trips after a certain interval, watch the timer and see if this occurs as the machine enters a given state. Another possibility is that the motor is ‘tired’, with partly deteriorated winding insulation.

If the breaker holds but the machine is still unresponsive, several possibilities must be considered.

After checking the supply wiring, you can remove the access panel and check voltage at the input terminals – and at the power light, timer, printed circuit board and motor terminals.

Perhaps an easier approach is to begin with the door switch. Just about all appliances have a door or interlock switch. Its purpose is to make sure the power is cut off when the door is opened so that the user or an inquisitive child will not be injured.

The switch can fail, so as to be always closed (no protection) or always open (appliance will not run).

The switch may fail electrically or its linkage may be worn or out of alignment. Either way, the mechanism should not be disabled except temporarily for test purposes, as serious injury can result.

Putting a little extra pressure by hand on the door will often make the dishwasher spring to life. If this is the case, the repair is simple.

A further test, with the door open, involves depressing the switch actuator by hand to see if the machine powers up. However, be extremely wary of injury without the protection of the interlock switch.

Still another test, with power disconnected, is to check the switch with a multimeter set to a low range in the ohms function. The reading with the switch open should be infinite ohms, and with the switch closed it should be near zero.

The switch is also defective if, while the mechanism is jiggled, the resistance is seen to fluctuate erratically. Switch replacement is a simple matter.

If any of the above measures don’t restore operation, it is necessary to go a little deeper. Check the water temperature and pressure. At the inlet there could be a strainer, which may be cleaned by blowing out in the reverse direction.

Check the timer to see whether one of the functions is not operating.

The entire timer is often bad. In this case, with power disconnected, remove the timer and shift one wire at a time over to the replacement timer, being certain to hook the power supply and all terminations correctly.

These connections are usually of the spade push-on variety, which makes the changeover easy. Use needle-nose pliers.

Most dishwashers are prevented from overfilling by a float switch. If the float is stuck in the up position, water will not flow and everything comes to a halt. Usually, repair consists of cleaning debris from around the float.

Another possibility is that the float switch has failed electrically. With power disconnected, the switch can be checked with an ohmmeter, working the float up and down manually.

The heating element could be burnt out, or it may not have voltage at the terminals. Check it with the multimeter.

Most dishwashers have a water pump under the sprayer arms, and the parts may need to be cleaned. Worn O-rings and seals are easy to replace. The sprayer arms have holes that may be clogged by debris or mineral deposits. They are easily cleaned using a sharpened hardwood dowel.

Some problems are more elusive and may be specific to an individual make or model. It is usually possible to get a schematic and service information from the internet.

Appliance technicians are a talkative lot, and online forums are fertile ground. You should be able to type in an exact description of the problem and find an answer in an existing archive or when an eager participant jumps in.

YouTube videos are excellent sources of information for tough repairs.

If it all comes down to the circuit board, that can be a bit of a problem. Prices vary widely. If you replace an expensive circuit board and the machine fails to work because you missed something simple, it may take some time to restore your image – but a lesson will have been learnt.