Rheem Furnace Light Blinking? What the Code Is Telling You

Count the flashes between pauses — that number is the furnace's own diagnosis. Rheem and Ruud (same family) blink their status through the little window in the lower door, and reading it takes one minute.

Safety first, every page: if you smell gas, don't reset anything. Leave the house and call your gas company from outside.

Read it right, then write it down

• Watch two or three repeats of the pattern to be sure of the count.
• Write down the code and the model number (sticker inside the cabinet) before any reset — cutting power clears the code.
• The legend printed inside your furnace door is the authority. Below are common meanings across many Rheem/Ruud models, but boards differ.

Common Rheem/Ruud codes

• Steady on or slow steady blink — normal operation on most models.
• 1–2 flashes — on many boards, ignition trouble or lockout: it tried to light, failed repeatedly, and quit. Often a dirty flame sensor. A power cycle usually buys one more attempt.
• 3 flashes — pressure switch fault: the furnace can't confirm its exhaust is drawing. Check the outdoor vent pipes for snow, ice, leaves, or nests — free and takes a minute.
• 4 flashes — high limit: overheated. The classic cause is a clogged filter or blocked vents. Filter to the light; can't see through it, replace it, wait 15 minutes.
• 5 flashes — flame sensed out of sequence. Shut it down and call.
• 6+ flashes — blower, grounding, or polarity faults. Pro territory.

One reset, then call smart

Fix the free stuff first (filter, vents, outdoor pipes), then reset once: furnace power switch off 30 seconds, back on. Same code again? Stop resetting. You now have the exact two facts a good shop wants on the phone: the code and the model number.

Related symptoms

• Furnace won't turn on
• Furnace short cycling
• Furnace blowing cold air

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