Furnace shows 12 flashes after power interruption; may not start immediately after power is restored.

Safety: Electrical measurements should be done by qualified persons. Turn off power before tightening connections.

Homeowner checks:

• Check breaker and wiring: Ensure the furnace breaker is fully on and not partially tripped. Inspect visible wiring to the furnace for damage.
• Avoid extension cords: Furnaces should be hard-wired or on a dedicated receptacle, not powered through extension cords.
• Power cycle: Turn off the breaker for 1 minute, then restore power. Wait several minutes to see if the furnace resumes normal operation.

When to call a pro: An electrician or HVAC technician should measure line voltage and transformer output. If low voltage is confirmed, they may need to correct supply issues or replace the transformer.