I have the afore mentioned Whirlpool Duet. It’s displayed the F20 error before and randomly stops during the cycle. It may run for 10 cycles w/o an error then it will stop. For the most part it does not display the F20 error. When accessing the diagnostics, using the 4 second button press jig, F20 is the only error to be seen. First checked that the hoses, screens, and supply valves were OK. I replaced the least expensive culprit being the flow meter but got no joy. It seems like the next step would be either the 3 inlet valve cluster or pressure switch would be next. Any guidance on where to turn next? It almost always starts but stops somewhere during the cycle.
Can the stored codes be erased from memory?
Do you have the HOT and COLD water valves turned to the on or fully open position?
Are the washer water supply hoses bent or kinked?
Do you have periods of low water pressure in your home? Pressure below 11 PSI may cause this error.
Are the water inlet valve filter screens (where the hoses connect to the washer) blocked and need to be cleaned?
Are you using the correct HE High Efficiency washer detergent? The wrong detergent can cause to much suds and give this error intermittently.
Try a deep cleaning cycle using one or two tablets of AFFRESH.
Clean the drain filter / trap. The video in this link can show you how to access that filter. Be prepared for some water to dump out when opening and cleaning this filter!
Try all this and give your results back.