What can you record safely?
If the light is visible through the normal viewing window, note its pattern without opening the running furnace. Count groups and pauses, not only individual flashes. Keep a photo or short video if it is safe to make one. Get the model from an existing invoice or accessible equipment label; have a professional identify it if cabinet access would be needed.
Use the owner's documentation or ask the installer for the exact chart. Do not disconnect power just to stop the blinking. That may leave the original question unanswered. Do not remove a panel, defeat a door switch, or reset a protective control to see whether the light changes.
What changes the recommendation?
If the applicable chart identifies normal operation and there is no other concern, the light itself may need no repair. Ask for the chart explanation and keep it with the equipment record. If it identifies a fault, the fact that some heat still arrives does not close that fault. A qualified technician should interpret the message in the context of the furnace's operation.
We recommend tying any proposed repair to the decoded message and supporting finding. A blink is a starting clue. It should not become a heat-exchanger replacement recommendation without the relevant condition assessment.
Tell the service provider if heating is cycling unusually, rooms are staying cold, or there is a separate shutdown report.
The next step is simple: ask, “For this exact model and this blink pattern, what does the manufacturer say, and does any finding need attention?”