If the service light on a 2000 Series heater is illuminating, it indicates the upper core charging high limit switch is open.
The core charging high limit switches are located inside the back panel and extend across the length of the heater. These limit switches monitor temperature between the steel back panel and the insulation panel within the heater. In most applications, if the heater's service light is illuminated, the heater is overcharging and will be hot to the touch.
If the service light repeatedly comes on, verify that the heater clearances have not been violated and that objects have not fallen between the heater and the wall. If clearances are correct, it is possible that the heater is overcharging or the back insulation panel has been damaged.
On the 2000 Series, overcharging is generally caused by one of two things:
Element Control Relay: If one of the element relays on the processor control board is stuck on, the heater will continue to charge even after the heater is up to temperature. The heater will then run on limit, meaning that it will charge until one of the core charging high limits trips. This will cause overheating of the core and can lead to element failure.
High Limit Switch: If one of the core charging high limit switches fails, it will open and not allow the heating elements in that section of the heater to charge. For example, if the lower core charging high limit opens, the lower two elements will be de-energized, but the upper two elements will continue to operate to get the brick core up to temperature. As the core temperature sensor sits between the upper and lower sections of the core, the upper section may overheat due to the lack of heat in the lower portion of the core. This will cause the upper high limit switch to open. This core imbalance can cause overheating of the brick core and can lead to element failure.