Common failure with age, or due to corrosion. Generally replaced when a major heater service is performed, along with the pressure switch. Should be replaced along with the Pressure Switch.
Raypak Gas Heater Error "LL" is often caused by the failure of these. Also failure may cut out power to the Circuit Board completely, depending on the model. This can cause RP2100 gas heaters to show a completely dead display. AKA Thermal Cutout
Raypak 600893b Hi Limit Switch - 140 F / 60C
Other RP2100 parts listed here: Raypak RP2100 Gas Heater Parts