Could be 1 of a few things really...depends on what brand controller i'd say...just my own opinion there..
So, check these out so you'll have something to work with,
Models with KD21C or KD218 Displays Error codes may only be on the display briefly. If you experience an issue, safely glance at the display to see if there is an error code. If the information below does not resolve the issue please contact sup...