Rheem furnaces use fault codes to indicate ignition and flame safety issues. Code 13, Flame Lost, signals that the furnace detected a flame failure during operation and could not confirm a stable flame as commanded by the control board. This guide explains what code 13 means in Rheem systems, common causes, safe diagnostic steps, and practical actions homeowners can take before contacting a technician. Understanding the code helps communicate effectively with HVAC professionals and minimize downtime during cold weather.

What Does Rheem Code 13 Flame Lost Mean?

Code 13 indicates the primary control did not detect a reliable flame within the ignition sequence. In most Rheem units, the gas valve opens and the igniter or spark is energized to establish a flame. If the flame sensor does not confirm a flame within a few seconds or if the flame goes out, the control locks out and the furnace stops gas flow to prevent unburned fuel release. This safety mechanism protects occupants from gas accumulation and carbon monoxide buildup. Flame Lost can stem from a faulty ignition, a dirty sensor, or problems in the gas supply or venting system.

In some Rheem models, Code 13 is also triggered by intermittent sensor signals, wiring faults, or a failed control board that misreads the flame signal. While some units cycle through resets automatically, repeated Code 13 occurrences usually indicate a persistent problem that requires inspection by a qualified technician.

Common Causes Of Flame Lost In Rheem Furnaces

Dirty or faulty flame sensor reduces the ability to confirm a flame, triggering a Code 13 even if ignition occurs.
Ignition device failure (glow bar, hot-surface igniter, or spark module) may fail to produce a stable flame.
Inadequate gas pressure or gas valve issue prevents sustained flame once ignition occurs.
Clogged burner or dirty burner surfaces impede flame stability and heat transfer.
Venting restrictions or exhaust problems can cause abnormal pressure switch readings and shut down the flame.
Drafts or improper combustion air disrupt flame stability, especially in tight or poorly vented spaces.
Electrical wiring or control-board faults may misread sensor signals or fail to energize the ignition sequence correctly.
Condensate drain or pressure switch issues can falsely indicate a flame issue if the venting system isn’t draining properly.

Step-By-Step Diagnostic Process

Safety is the first priority. Before inspecting any components, switch off the furnace at the thermostat, shut off the gas supply if accessible, and cut power at the service panel for the unit. Do not attempt to bypass safety features. The diagnostic steps below are intended for informed homeowners and HVAC professionals.

Visually inspect the ignition area, burner, and flame sensor for signs of damage, corrosion, or residue. A dull or corroded flame sensor reduces signal strength.
Check the gas supply and regulator pressure to ensure the gas valve opens properly and delivers a steady flow during ignition.
Test the ignition device. If the spark is weak or the igniter does not glow, the ignition module or igniter may need replacement.
Observe the flame sensor during ignition. A clean, bright flame is required; a blue flame with a steady shape indicates proper combustion, while yellow tips or flickering flames suggest improper gas mix or dirty burners.
Inspect burner orifices and burners for dirt, spider webs, or debris. Clean as needed with appropriate tools or consult a professional for safe disassembly.
Check venting and the pressure switch circuit. A blocked vent or cracked exhaust can prevent proper venting and trigger a safety lockout.
Examine control wiring and connections to the ignition module and flame sensor. Loose or damaged wires can produce intermittent signals and faults.

Note that some steps require specialized tools and expertise. If a homeowner is unsure about gas appliances, it is prudent to call a licensed HVAC technician rather than proceeding with advanced tests.

Safety First: Handling A Flame Lost Condition

Gas furnaces involve flammable fuel and combustion byproducts. If a gas smell is detected, leave the area immediately, avoid creating sparks, and call the gas company or emergency services. Never bypass safety controls or disable the flame sensor to run the furnace. Carbon monoxide is odorless and dangerous; a CO detector should be installed and tested regularly. When in doubt, err on the side of caution and contact a professional.

Homeowner Troubleshooting Checklist

Turn off the thermostat and reset the furnace by cycling power: switch the thermostat off, reset the breaker, wait 60 seconds, then restore power. Repeat once or twice to see if the fault clears.
Clean the flame sensor: power off, remove the sensor probe carefully, and gently rub with fine steel wool or a non-abrasive pad. Reinstall and test ignition.
Inspect and clear burner and flame path obstructions; ensure burner surfaces are clean and free of debris.
Verify gas shutoff valve is fully open and listen for a steady gas flow when the valve opens during ignition. If gas pressure seems low, contact the utility or a technician.
Check air filters and blower compartment for obstructions that could cause improper combustion air supply or heat transfer.
Inspect venting for blockages, moisture, or improper slope. A blocked vent can trigger pressure switch faults and flame loss.
Ensure thermostat signals proper heat demand and there are no mismatched wiring conditions between the thermostat and furnace control board.

Resetting Rheem Furnace After A Code 13

Resetting can sometimes clear a transient fault, but persistent Code 13 requires addressing the underlying cause. Follow these steps carefully:

Turn the thermostat to Off and set the furnace switch or breaker to Off for about 60 seconds to drain residual power.
Return the power to On, then set the thermostat to Heat and a comfortable temperature. Allow the furnace to attempt ignition and run through a cycle.
If the code reappears after a complete cycle, perform the cycle again. If Code 13 recurs, do not continue resetting repeatedly; this indicates a fault that needs professional diagnosis.

Some Rheem models include a dedicated reset button on the control board. If present, press and hold according to the manufacturer’s instruction for a brief period, then observe the system during startup.

When To Call A Licensed HVAC Technician

Code 13 requires professional assessment if any of the following apply: repeated flame loss after resets, a strong gas odor, persistent furnace shutdowns during cold weather, or the ignition sequence producing weak or no spark. A technician will perform a thorough safety check, test gas pressure, inspect flame sensor integrity, verify control board operation, and diagnose venting and condensate issues. Timely service can prevent unsafe conditions and extend furnace life.

Parts Involved In Flame Lost Scenarios

Understanding common replacements helps homeowners evaluate estimates and repairs. The following table outlines key components, their role, and typical symptoms when faulty.

Part	Function	Common Symptoms When Faulty	Notes
Flame Sensor	Detects flame presence and signals control	Weak signal, intermittent flame confirmation, Code 13	Can often be cleaned; may require replacement if corroded
Ignition Device	Ignites gas (spark module or hot-surface igniter)	No ignition or delayed ignition	Failure often requires professional replacement
Gas Valve / Gas Regulator	Controls gas flow to burners	Inadequate gas flow, lingering odor	Pressure should be tested by tech
Burners	Provide flame for combustion	Clogged or dirty burners cause unstable flame	Cleaning or replacement may be needed
Control Board	Coordinates ignition sequence and flame sensing	Communication faults causing misreads	Board diagnostics often require professional tools
Vent System & Pressure Switch	Ensures proper venting and safe operation	Blocked vent or failed pressure switch	Fix requires clearing vent or replacing switch
Thermostat Wiring	Signals heat demand to furnace	Incorrect wiring causing fault signals	Check connections and compatibility

Preventive Maintenance To Avoid Flame Lost

Schedule annual professional servicing to inspect ignition, flame sensor, gas pressure, and venting.
Replace air filters regularly to maintain proper airflow and combustion efficiency.
Clean or replace the flame sensor and verify its grounding connection during routine service.
Keep the venting clear of debris, lint, and blockages; ensure proper vent termination and slope.
Test and verify gas pressure as per Rheem specifications; avoid improvised adjustments without proper equipment.
Inspect electrical connections for corrosion, loose terminals, and damaged insulation; address wiring issues promptly.

Additional Resources And Warranty Considerations

Consult the Rheem user manual and model-specific service guides for code interpretations and reset procedures. If the furnace is under warranty, follow authorized service channels to preserve coverage. Authorized technicians can provide precise diagnostics, safe gas handling, and correct replacements with OEM parts. For homeowners, keeping documentation of service visits, sensor replacements, and parts used can help with warranty claims and future maintenance planning.

Frequently Asked Questions

Q: Is Code 13 dangerous to ignore? Yes. A flame loss fault indicates potential gas leakage and carbon monoxide risk; do not bypass safety features or leave the furnace running in this condition. Contact a technician for diagnosis and repair.

Q: Can I fix Flame Lost myself? Minor cleaning of the flame sensor can be a DIY step, but ignition, gas pressure, and control-board issues require a licensed professional due to safety concerns.

Q: How long does a typical repair take? It varies with the fault; simple sensor cleaning may be quick, while diagnosing gas pressure, venting, or control-board faults can require a few hours.