Common problems with furnace relay switches can involve faulty thermostat wiring, tripped circuit breakers, or unlit pilot lights. Symptoms of a malfunctioning furnace may include rattling noises and poor heat regulation, which can also result from a blown fuse.
If a furnace fails to heat, its circuit breaker may be turned off. The circuit can be examined through the main electrical panel, then reset by shutting it off once and turning it back on. Additional trouble with a short circuit could be a sign of a short in the furnace's electrical system. A faulty thermostat can be stabilized by adjusting its heat anticipator to regulate the furnace cycling system