Ferroli Optimax HE31C Error Code NOcn?

I have a Ferroli Optimax HE31C Boiler and it has been working fine for years but all of a sudden it has fault NOCN... The Heating still pumps around the house but it doesn't seem to be talking to the Thermostat, it just pumps constantly and heats up to the water temp set on the boiler. The hot water also works but the flame size on the boiler seems inconsistent (fires high then low in quickly back and forth)... Don't know where to start, tried banging the diverter valve as other suggested but that seems to be working fine... PLEASE HELP......