Flame Monitor Sensor - EPX, MPX
EPX-2-22, MPX-2-30

The flame monitor detects if the unit is working, if there is a flame. 
If the flame monitor doesn't see a flame after 25 seconds of operation or the lens is blocked from seeing a flame because of dirt or dust, it will cause the system to shut down. 
Regular maintenance may require cleaning the lens of the flame monitor. 
If replacement is required the monitor housing is held in position by two screws and the monitor is pluged into the circuit board. 
There have been reports of units not operating if the unit is facing directly into the sun