My oil fired boiler (de Dietrich GTU 200) has the alarm bell flickering. On the 4 radiator setting it doesn't turn itself off until it reaches 50degrees. The temperature continues to rise to 56degrees until it begins to cool. It then fires up again and alarm light comes in. It showed AL this morning on the temperature dial. Anyone know what is happening. I'm in France and the company I used for the service in useless.

Ovens - De Dietrich