[Elecraft] K2: Alternating c/F in display

[Fred Jensen]

My K2 has suddenly begun flashing a "F" in the rightmost position of the 
display.  It lasts about a second and then reverts to a "c" for about 2 
1/2 seconds and then repeats. What is it trying to tell me?

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County