Rheem Furnace 1 flash: What It Means & How to Fix It
Compiled & reviewed by the HVAC Flash Codes editorial team · Updated 2026-06-25 · confidence: high
⛔ Stop — call a licensed pro
What does Rheem furnace 1 flash mean?
Soft lockout (about 1 hour) — no flame was sensed after four ignition trials; the control locks out and will auto-retry. Most often caused by Dirty or failed flame sensor.
What your furnace LED does. The diagnostic light blinks 1 flash, pause, repeat — that pattern is Rheem furnace 1 flash.
What Rheem furnace 1 flash means
Soft lockout (about 1 hour) — no flame was sensed after four ignition trials; the control locks out and will auto-retry.
Common causes
Dirty or failed flame sensor
No or insufficient gas supply to the burners
Igniter or ignition-circuit failure
Burner or gas-valve fault
Homeowner-safe steps to try
Confirm the gas supply is on (other gas appliances work) and the furnace gas shutoff is open
Replace a dirty air filter and clear supply/return vents
Try one reset by lowering the thermostat, waiting, then calling for heat, or cycle furnace power once
Do not clean the flame sensor, adjust the gas valve, or touch the burners
When to call a licensed HVAC technician
If the basic checks below don't clear the code, the fault recurs, or anything involves the gas valve, burners, flame sensor or wiring, stop and book a licensed HVAC technician. Repairs to the combustion or gas system are not DIY.
⚠️ Safety first. Gas furnaces burn fuel and produce combustion gases. If you ever smell gas (a rotten-egg or sulfur odor), hear hissing, or your carbon-monoxide alarm sounds, do not touch light switches, thermostats, or the furnace — leave the home immediately and call your gas utility's emergency line or 911 from outside. Homeowners should limit themselves to simple, safe steps: setting the thermostat, checking and replacing the air filter, confirming the furnace switch and breaker are on, keeping supply registers and return grilles open, and clearing snow, ice, or debris from outdoor vent and intake terminations. Never bypass, jumper, or reset safety switches (limit, flame-rollout, or pressure switches), never open the gas valve or burner compartment, and never attempt combustion, electrical, or control-board repairs yourself. Error codes point toward a problem but are not a substitute for proper diagnosis — when in doubt, turn the system off and call a licensed HVAC technician. Keep working carbon-monoxide detectors on every level of your home.
When to call a pro: Call a licensed HVAC technician if it locks out repeatedly — flame-sensor, igniter, gas-valve, and burner work require a pro. If you smell gas, leave the home immediately and call your gas utility or 911 from outside; do not flip switches.
Rheem furnace 1 flash FAQ
What does Rheem furnace 1 flash mean?
Soft lockout (about 1 hour) — no flame was sensed after four ignition trials; the control locks out and will auto-retry.
What causes Rheem furnace 1 flash?
Common causes: Dirty or failed flame sensor; No or insufficient gas supply to the burners; Igniter or ignition-circuit failure; Burner or gas-valve fault.
Can I fix Rheem furnace 1 flash myself?
You can safely try: Confirm the gas supply is on (other gas appliances work) and the furnace gas shutoff is open; Replace a dirty air filter and clear supply/return vents; Try one reset by lowering the thermostat, waiting, then calling for heat, or cycle furnace power once; Do not clean the flame sensor, adjust the gas valve, or touch the burners. Call a licensed HVAC technician if it locks out repeatedly — flame-sensor, igniter, gas-valve, and burner work require a pro. If you smell gas, leave the home immediately and call your gas utility or 911 from outside; do not flip switches.
Sources: my.rheem.com · my.rheem.com. General information for homeowners, not professional advice — confirm against your furnace's manual and a licensed technician.