Fenwal Model 550 is a digital temperature controller designed for industrial process monitoring and control, typically interfaced with Type J thermocouples or RTDs and offering programmable setpoints up to 800 °F. Common failure points include burned-out relay contacts, corrupted firmware, or degraded sensor input due to wiring corrosion; the unit may also exhibit erratic output when the power supply is unstable or when the internal temperature probe has been physically damaged. First‑check steps: verify that the controller receives its rated 24 VDC (or 120/240 VAC as specified) and confirm proper grounding, inspect all input/output wiring for continuity and secure connections, then review any stored error codes or LED status indicators before proceeding to component replacement or firmware reflash.[oss-gpt-20b]