From Adam Hintz
The F21 error on this line of washers is the machines way of saying the water took to long to drain from the tub. This could be a problem with one of the washer parts including the drain pump itself, how the drain hose enters the stand pipe, or maybe even something is stuck somewhere in the drain system and is preventing the water from flowing properly. Not quite an emergency, but it is an indication of a problem and a problem that can lead to something bigger if not looked into.
From Old Turkey on SUD - According to the service manual there are a few possibilities
Sud - SUDS LOCK (Overdose of Detergent detected during the Wash Cycle)
The motor control unit senses a suds lock condition by analyzing the current draw on the drive motor. If “Sud” is displayed a potential suds lock is detected. This may signify a bad pump, an extra heavy load, excessive detergent, or excessive suds.
• If too much detergent was used, run the unit through a Rinse/ Spin cycle then a Normal cycle without adding any detergent. This should clear the unit of the excess detergent.
• Check the drain hose and make sure it is not plugged or kinked
• Check Wire Harness connections to the Drain Pump, Pressure Switch, and Central Control Unit (CCU)
• Check/ Clean Drain Pump Filter of foreign objects
• Check Drain Pump
• Check the Pressure Switch
• Check CCU
To make your job easier, try to download the manual from here. Good luck and I hope it helps.