Washing Machine Sud & F21 Error Code
This video shows the final fix first, followed by the long journey to figure that out. Thus don't go right to the end for the fix. It was a plugged Water Level Pressure Switch Air Trap that just needed cleaned or replaced for like $20.
Kenmore Elite QuietPak4, HE3t, Model 11049962603 (~2009 model). Sud error code along with the F21 error code, but it wasn't the Drain Pump, plugged strainer, plugged hose, or the pressure switch. It was a plugged Water Level Pressure Switch Air Trap that just needed cleaned or replaced for like $20.