Common problems with gas boilers include temperature/pressure gauge failure, pilot light failure and trouble with the circulatory pump. Some of these problems can be repaired without professional assistance.
The temperature/pressure gauge is located on the body of the boiler. Typically, these devices have a red line. Properly functioning boilers have a reading above zero, but below this red line.
If the boiler has trouble, but the temperature and pressure are reading normal, the problem is likely air in the system. This is common with boilers featuring the old style of manual air eliminators. The best fix for this problem is replacing the old manual system with a modern float system. If the temperature and pressure are low, open vent dampers should be checked to see if the inducer fan is running. If either of these are not occurring, turn the gas boiler off and back on.
When the pilot light does not light, the vent pipe may have become blocked. If the pilot attempts to light, but fails to stay lit, the assembly itself might be dirty. This can be cleaned without seeking professional assistance.
The circulatory pump pushes the hot air around the building. If the pump is not running, check the power supply. If the motor feels too hot, the run capacitor may need to be replaced.