Separate controller demand, protection settings and output behavior.
Electrical access and testing are for a qualified electrician. Isolate all involved circuits and verify they are de-energized before opening boxes, disconnecting conductors or measuring resistance. Energized voltage measurements require appropriate electrical safety precautions.
For the SCP-120 / ETI APS-3C, check the Power, Snow, Heating and High Temp indications, operating switch and Hold-On Time before diagnosing a stuck relay. A hold-on period can intentionally keep heating active after snow detection stops; high-temperature protection can prevent heat even when it is requested.
Compare the controller’s documented demand state with the actual output and downstream relay-panel operation. If they disagree, a qualified electrician should check supply, wiring, protection and the load circuit. An indicator alone is not proof of voltage or current at the heater.
If water, corrosion or damaged electronics are present, keep the affected controller de-energized and identify the ingress source before restoring service. Do not bridge relay contacts, bypass high-limit protection or install a generic replacement relay based only on its coil voltage.
Use the exact controller revision and manufacturer-approved service decision. The old generic relay part number is not established as an approved replacement by this manual.
Source: SCP-120 / APS-3C manual (Operation, Hold-On and High-Limit Functions).