The service light is a line voltage neon light that is wired directly across the heater's core high limit switch. Repeated illumination of this light would indicate a need for service. The light itself rarely fails and if the contacts are closed on the core charging high limit switch when the heater is cool and there is no mechanical damage to the part it is almost always within calibration.
If the service light is illuminated and the heater is cool to the touch, check to see if the core charging high limit switch is open. If the service light is illuminated and the heater is hot to the touch, check for proper operation of the heating elements, the control board, and the brick core temperature sensor.
NOTE: This part is no longer available. Contact the Steffes Sales department for information.