Hey there guys. I need some help! i'm working on a Carrier Transicold freezer container running on R134a. I keep getting an error code (AL21) on the electronic controller & the unit refuses to start up.The alarm LED is continuosly lit. Earlier this week it had an "AL52" fault & we traced that back to a fault evap. fan. I suspect AL21 to be one of the sensors but thats just a guess. Anyone with solid knowledge on these error codes could please help. I would appreciate it, if i could get the full list of codes for this machine. Thanx.