Freezer evaporator frosted fan working at all .

Freezer evaporator frosted fan working , replaced main board told customer to leave ref off to defrost evaporator she did that. customer call back a few days latter refrigerator warm freezer was working fine. I stopped out to check freezer -2 degrees ref 60 . Freezer fan was not running compressor was running . Shut refrigerator off to check make sure compressor cycled off. when i turned tit back on evap fan started to run. customer told me she only left it off for 6 hours. Not sure if fan may have been frozen, called her back next day, my tech went back with a new board and replaced 2nd board he said the fan was running when he got there . Is there anything that would stop the evaporator fan from running. or is this one of the fans if it goes bad can take out the board fan was not replaced yet