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Scrape away large pieces of food, bones, pits, toothpicks, etc. Burned on food should be loosened before loading. Empty liquids from glasses and cups. Foods such as mustard, mayonnaise, vinegar, lemon juice and tomato products may cause discoloration of stainless steel and plastics if allowed to sit for a long period of time. Unless the dishwasher is to be operated at once, it is best to rinse off these food soils. Load racks so that large items do not prevent the detergent dispenser from opening. If the dishwasher drains into a food disposer, be sure disposer is completely empty before starting the dishwasher.
The first thing you must do is figure out what style of d/w you have and a little of how it works so you can have a better understanding of what to look for. There are two basic designs, the direct drive dishwasher and the forward-reversing dishwashers. Direct drive dishwashers do not have to stop to drain the water will forward-reversing dishwashers have to stop the motor and start it up in the other direction to drain the water. This is important if you have a no draining problem, knowing more about your d/w will help you find the trouble maker quicker and easier. For more draining help, see here. For more washing tips, see here.
Check to make sure which style d/w you have as outlined above. The direct drive dishwashers have a drain solenoid attached to the motor and pump or a separate drain valve that must be checked with a ohm or volt meter. The drain solenoid and valve energize and opens the drain port to allow the water to pump out. For a "no drain" problem, always start with the common stuff first...a kinked or clogged drain hose. The drain hose must be checked and cleaned out and if you have to, remove the drain hose(s) to fully check and clean them. Don't forget the drain system sometimes consists of a self cleaning body that also can get clogged and stop the dishwasher from draining. If your drain hose needed to be lengthened a hose connector was probably used, these hose connectors are slightly smaller than the drain hose and food bits often get clogged there, also the tail pipe piece that connects the drain hose from your d/w to the house drain is also another popular place to get a clog. On the forward/reversing style of dishwashers that won't drain, make sure the motor is reversing to drain the water out. If the motor does not reverse to drain, the motor, wires ( especially where the doors bends to open and close ) and timer will have to be checked. If the motor is reversing and running, the pump impellor inside the motor and pump assembly may be damaged and is incapable of pushing the water out to drain. If the drain solenoid or drain valve checks ok on the direct drive motor and pump, the impellor inside may also be damaged as mentioned above.
Always check the drain spout under the sink for a d/w that will not drain, also if the Air gas is leaking ( see Fig1A ) this is a clogged air gap or drain and normally not a d/w problem. These are some typical drain hook ups....if you have a drain question, may be a good idea to refer to one of the figures so we all know which style you have :-)
Some common things to check : Water temperature - Use a thermometer inside the dishwasher to measure the water temp. it should as close to 140 degree F as possible, try using the temp boost feature if your d/w is equipped with this cycle feature to see if this will help your washability problems. Powder soap - Make sure you store your soap in a dry environment and that it is not stale and clumping together. Proper filling - Most dishwashers will fill with water up to and around the heating element, if water is way below the heating element you may have low water pressure or a problem with the fill valve. For low water pressure, try to clean the screen at the entrance of the fill valve, sometimes the fill valve can fail and not allow the proper water into the dishwasher and may need to be replaced.
Low water fills are a very common trouble maker with today's dishwasher!! Dishwashers use so little water compared to ones 10-20 years ago that just a little less water from the fill cycle can/will effect it's ability to wash the dishes properly.
Dishwasher fill valves also have a screen built into the valve entrance, this can also get all clogged up and need to be cleaned.
Make sure the spray arm(s) are turning freely, with water inside the d/w you can make a mental note where they are and turn on the d/w for a couple of seconds to see if they are actually moving properly. Things that can effect the operation of the spray arms...plugged holes in the spray arm...worn holes in the bottom of the spray arm...split or melted spray arm...the supports, gaskets and washers that hold the spray arms can wear out and leak water around the spray arm rather than allow the water inside the spray arm. The supports can also bind and slow down the spray arm enough to effect the washing of the d/w. Some sample supports...1one, 2two, 3three, 4four - Some sample gaskets...1one, 2two, 3three...
Often this is a form of hard water filming, soak a glass with the white filming on it, in a bowl of undiluted vinegar for approximately 1/2 of an hour and wash the glass with some soap and rinse under the tap in hot water. If the white marks disappeared after soaking in vinegar and then washing, it was hard water filming. I have some good reading on hard water filming...here...and the finish is...here.
Sometimes the shape of the object ( like a baby bottle with a small opening on the top ) does not allow full water pressure inside and you only get some steam, this can leave salt and pepper like junk inside. Water temperature, and condition of granular soap will also have to be checked. I have some good reading on this subject...here...and...here.