Ravenheat CSI 120 error code 04?

I have a Ravenheat CSI 120 on which I recently (last week) replaced the diaphragm in the diverter valve. I then vented the system, as best I could, by bleeding the radiators. Everything worked fine until this morning when the heating cut out, both CH and DHW, and the boiler showed an 04 error code, which is overheat lockout. After that the boiler would not start and continues to display the 04 code. I checked the overheat thermostat with a test meter and got continuity, which I assume is as it should be, and I am now a bit flummoxed as to what to do next. Could it be that I have not properly vented the boiler after removal and replacement of the diverter valve? Any help would be appreciated.