Fault showing ef?

Hi, I put the downstairs alarm on last night, then I had to go downstairs again more or less straight away. When I entered the kitchen, the alarm didn't go off but the Ef was flashing. I don't know if returning fairly quickly cause the unit to malfunction or not. There is nothing in the manual indicating how to fix it, just what the code means. Does anyone have any idea how I can fix it? Thanks.