I suspect one of the temperature sensors has failed. I'm not familiar with this boiler but it will have two or maybe three sensors, they are easy to find - they are screwed into bosses on the pipework, each with a two pin plug connector. Use a multi-meter on ohms range to check them; typically they'll read from say 5K up to maybe 20K cold, this should reduce steadily as temperature rises so they'll read maybe 1K at boiling point. The chances are that all your sensors are identical which should help you to determine the failed one. Hope this helps.
Frank, January 2009
