Alpine 105 soft lockout 17

[argyle87]

I have a 3 or 4 year old Alpine 105 that provides domestic hw and supplies 2 radiant circuits. Two days ago, after switching on one of the radiant loops, i had a high limit error. I replaced the limit sensor, and the Alpine ran for 24-36 hrs. Tonight I have "soft lockout 17 - fan speed not proved", and no ignition. I am frustrated and desperate for help. Any/all suggestions GREATLY appreciated.