A solid-state relay can fail with no output, continuous output, intermittent switching, or excessive voltage drop. A failed-closed device can overheat a zone even when the controller shows zero demand, while a weak or overheated device may reduce delivered power under load.
The controller command, relay input, load voltage, current, heat-sink temperature, and switching pattern should be checked together. Current measured only at an average meter may miss short cycling or one missing phase. Electrical testing must be performed by qualified personnel under appropriate isolation procedures.
The replacement relay must match load type, current margin, control signal, switching method, and heat-sink requirements. Loose terminals and inadequate cooling can destroy a new device. Heater recipes should be reviewed after repair because previous settings may have compensated for reduced output.