FEC's seem to be the second check box to ES errors.
On my upload side I have slightly higher ES's but lower FEC's and thats not interleaved. But on my download side I have lower ES's than the upload, but more FEC errors, with an interleaving set to 1283.
It's been like that for 28 days.
I'm running an older firmware at the moment: A2pv6C035m.22g (with software version SP10). I intend on updating this soon. I'm just seeing how long the modem holds connection. 28 days is my record so far, previously the HH5 never got past 14 days lol.