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Can you give me the complete model number on the furnace? It will be on a label on inside furnace panel just above burners.Remove panel to burner compartment and look inside. The model number will help me give accurate assistance for your specific unit.
Below is a link showing where to find label with model number:
Let me see if I have a manual. The 21 blinks is normally just telling us blower is moving 2100 cfms of air).
Water heater not require near the gas the furnace will, Do burners seem to be as forceful as normal?
OK got the manual. Now the status light (shown in my manual as RED) should blink between 2 and 10 flashes
Please verify if these are the lights you are referring to and number of blinks on each