Furnace ignitors burn gas on the main burners. While the ignitor does not provide significant heat on its own, it is the primary component that allows burners to work properly; and when the ignitor fails, the burner will not work. Most modern ignitors are electric spark components that use electrodes to light the gas, oil or propane that your furnace uses. In many cases, a cracked ignitor is caused by electrical or corrosion problems.
Ignitors are one of the weak points of burners and can be damaged by corrosion from the surrounding air. The acidic condensation that results from combustion does not create the best environment for the relatively weak metal parts of the ignitor. Corrosion can stop the ignitor from creating sparks, and eventually cause the ignitor to crack or burn out entirely.
Electrical Current Issues
A sharp burst of electricity, or current mismanagement via a problem in the furnace control panel, can cause an ignitor to crack as well. Ignitors were made to handle a certain current range. If the current veers too far outside this range, the ignitor may crack, especially if it has protective ceramic pieces. In this case, the problem can recur because it lies with the control panel and current, not the ignitor.
Mechanical malfunctions occur when the ignitor physically breaks. This occurs when it is damaged during other repair work, or when the ignitor is misaligned and damaged through furnace vibration. If vibration is slowly ruining your ignitors, consider re-mounting your furnace in order to prevent any other internal components from being damaged.
A cracked ignitor cannot be fixed. The solution is always to replace the ignitor entirely. If you are not sure that the ignitor is actually broken, you can test it with an ohmmeter for resistance to see if an internal crack exists. This may also help you narrow down an electrical or wiring issue in your furnace that is causing the ignitor to crack repeatedly.