JET TECH/model F18/small amount of water stays in bottom.Wont fill or cycle but water runs.

Check the inlet screen to the water circulating pump and the water heater element for lime buildup. I've seen a heater element that had a 1/4" thick layer of lime encasing it. The inlet also had some lime and other debris clogging the screen. After carefully chipping away as much as was practical and running lime remover (such as CLR) through the machine, it heated the water much more quickly and had an effective spray.

To prevent a recurrence of this problem, make sure you have a functional water softener ahead of the water heater feeding the machine.

Verify the contactor for working condition. maybe burned wires or bad contactor. if it is good then the rinse thermostat or, less likely, the booster element.

The pressure swith (load size) is what controls that chang that. A small hose runs from the bottom of your tub and as the water fills a measerd amount of water fills that tube and pressure build up in their operates that switch and activates the timer to stop filling and start agitating.

The water-level switch regulates your washer's fill volume. This switch is usually a diaphragm device with a small, clear tube attached between the switch and the bottom of the washer's outer tub. As the water level in the tub increases, the pressure on the air in the tube increases. When the pressure reaches a certain level, it activates the switch, shuts off the water, and signals the timer to begin the agitate cycle. If the switch is defective, it may prematurely signal the water to shut off. If so, you probably need to replace the water-level switch.

try it on rinse cycle if goes ok then the hot water valve is faulty

I had same symptoms on a Hotpoint dishwasher.
Turned out to be a failed water heater.
The logic in this case goes like this:
When wash cycle starts, the first thing the machine does is pump out any water that has collected in the sump since it was last run.
It then allows a small amount of clean water in and enables the water heater.
If heater okay, it then continues with the rest of cycle.
In this case, the control board detected failed heater so pumps out the water and then stops with an error code shown by a flashing lamp.

No simple fix I'm afraid - machine had to be turned on its side, bottom panel removed, several items had to be removed to get at the water heater which was then repalced with a new one.

You dont state what make/model your dishwasher is so maybe worth adding this information as someone else may recognise your fault and have a quick fix.

first feel around the drain and see if there is something stuck inside the drain. then take a small amount of clorox2 powder and run it through a cycle. after that try jet dry. sometimes there is a grease build up or a lime build up that needs to be removed.

Solution: Just refill the water tank with water (no descaling solution) all the way to the "Max Line". Then run the full descaling program again --takes approx. 20 minutes. After the program runs completely you will get the yellow "standby" light on and the red "descaling" light will be off.

Next time you descale, be sure and fill your water tank to the "Max Line" before running the descaling program.
Hint: An incomplete cycle (not full tank) is what makes the red light stay on. In other words, it's staying red to tell you to run a full cycle (beginning with the water tank filled to the "Max Line"). Hope this tip is helpful!