There are plenty of models of gas boilers in commerce and each of them is exposed to more or less relevant issues. From this point of view, knowing how to observe problems is a first step in an efficient reparation. For example, most people have no clue what to do when the pilot light goes out. They do know that they have a problem, but they have no clue about the causes. Perhaps you have a defective thermocouple which stops the gas delivery to this light or maybe the light is blown up. According to most plumbers, a buildup can be a cause too.