Rheem Furnace Code 13: Flame Lost Troubleshooting and Fixes

The Rheem furnace error “Code 13 Flame Lost” indicates the burner flame extinguished unexpectedly during operation, interrupting heat delivery and often triggering safety lockouts. This article explains causes, step-by-step diagnostics, repairs, safety precautions, and when to call a professional to restore reliable heating. The goal is to provide clear, actionable guidance for U.S. homeowners dealing with this specific fault.

Symptom	Possible Cause	Likely Fix
Code 13 / Flame Lost	Dirty flame sensor	Clean or replace sensor
Intermittent flame	Weak gas pressure or valve	Check gas supply; call tech
No ignition	Igniter failure	Test and replace igniter
Frequent lockouts	Blocked vent/combustion air	Clear venting and intake

What “Code 13 Flame Lost” Means

Rheem furnaces use diagnostic codes to identify operational issues. Code 13 (Flame Lost) means the control board detected a flame during the ignition sequence or running cycle, then lost it unexpectedly. The system usually attempts a restart and then locks out after several failed tries to prevent unsafe conditions.

How The Furnace Detects Flame

Modern Rheem furnaces sense flame using a flame sensor or a flame rectification circuit. The sensor produces a small electrical signal when exposed to the ionized flame. If the control board does not see that signal during the expected period, it reports a flame-lost error and shuts the gas valve.

Common Causes Of Code 13

Several mechanical or environmental issues can cause a flame to go out. Dirty flame sensors, failing igniters, inconsistent gas supply, flame rollout, restricted venting, and control board faults are the most frequent culprits.

Dirty Or Corroded Flame Sensor

Residue from combustion accumulates on the sensor tip, reducing its ability to detect flame current. This is the single most common cause of intermittent flame detection and Code 13 lockouts.

Weak Or Faulty Igniter

If the igniter doesn’t heat reliably, the flame may light inconsistently or fail to sustain, causing the system to detect flame then lose it as combustion fails to remain stable.

Gas Supply Or Gas Valve Issues

Low gas pressure, a partially closed service valve, or a sticking gas valve can cause the flame to sputter or go out after initial ignition. These issues require careful pressure testing and service by a qualified technician.

Blocked Venting Or Combustion Air

Restricted flue vents or inadequate combustion air can cause unstable combustion and flame rollout, making the furnace shut down for safety.

Flame Rollout Or Heat Exchanger Problems

Combustion gases spilling into the cabinet (flame rollout) often due to cracked heat exchangers or clogged burners are serious safety hazards. They can cause flame loss and require immediate professional attention.

Control Board Or Wiring Faults

Corroded wiring, loose connectors, or a failing control board can interrupt the flame signal or misinterpret it as loss of flame, creating false Code 13 events.

Tools And Safety Preparations Before Troubleshooting

Before attempting diagnostics, turn off power to the furnace and shut the gas supply if instructed by a professional. Recommended tools include a multimeter, insulated screwdrivers, nut drivers, a soft cloth, steel wool or 400-grit sandpaper, and a flashlight. Do not attempt gas valve or internal gas system repairs unless qualified.

Step-By-Step Troubleshooting Guide

1. Record The Exact Code Behavior

Note whether Code 13 appears immediately at startup, after a few minutes of operation, or randomly. This timing can pinpoint whether ignition, sustained combustion, or heat-related issues are at fault.

2. Check Simple Things First

Verify thermostat settings, power to the furnace, and that the gas supply valve to the unit is open. Confirm the furnace door is properly seated; many units lock out if a safety door switch is open.

3. Inspect The Flame Sensor

Remove the flame sensor, inspect for black or white coating, and clean gently with fine sandpaper or steel wool until the metal looks shiny. Reinstall and test. Cleaning often resolves intermittent Code 13 faults.

4. Observe The Ignition Sequence

Turn the furnace on and watch the sequence: inducer motor, ignition, and flame establishment. If the igniter glows but flame doesn’t sustain, it could be a gas delivery issue. If the igniter fails to glow, replace it.

5. Check Burners And Combustion Area

Inspect burners for debris, rust, or blockage. Clean burners and pilot areas per manufacturer guidance. Ensure venting and intake openings are free of nests, leaves, or snow.

6. Test Flame Sensor Current With A Multimeter

With the furnace running, measure the microamp signal from the flame sensor or the control board input. Typical flame current is between 2–10 microamps. Readings below 1–2 microamps indicate sensor or grounding issues.

7. Verify Gas Pressure And Valve Operation

Gas pressure should match the furnace specifications. A certified HVAC technician should perform manifold pressure checks and gas valve diagnostics. Never attempt internal gas valve repairs without proper certification.

8. Inspect Wiring And Control Board

Look for loose, burnt, or corroded connections on the control board, flame sensor wiring, and gas valve leads. Replace damaged wiring and retest to rule out intermittent electrical faults.

When To Replace Parts

Replace the flame sensor if it is heavily corroded, broken, or cleaning does not restore proper signal. Replace igniter if it cracks, fails to glow, or shows open circuit on a multimeter. Replace burners if warped or badly corroded, and replace control board if diagnostics indicate board-level failure after ruling out wiring and sensors.

Costs And Time Estimates

Cleaning a flame sensor is low-cost if done by a homeowner or technician and takes 15–30 minutes. Flame sensor replacement ranges $60–$150 for parts and DIY labor; professional service may cost $150–$300. Igniter replacements typically run $100–$300 installed. Complex gas valve or control board repairs can cost $400–$1,200 depending on parts and labor.

When To Call A Professional

Contact a licensed HVAC technician for suspected gas pressure problems, flame rollout, cracked heat exchanger, repeated lockouts despite sensor cleaning, or any issue involving gas valve or wiring replacement. Safety risks from gas leaks and carbon monoxide make professional service essential in many cases.

Preventive Maintenance To Avoid Code 13

Schedule annual professional maintenance, including burner cleaning, flame sensor inspection, heat exchanger check, and venting verification. Change air filters regularly, keep combustion air intake clear, and ensure the area around the furnace is free of clutter to maintain proper airflow.

Safety Considerations And Carbon Monoxide Risks

Flame loss and combustion instability may increase carbon monoxide risk. Ensure working CO detectors are installed near sleeping areas and the mechanical room. If any unusual smell, soot, or yellow flames appear, shut the furnace off and call a technician immediately.

Rheem-Specific Tips And Resources

Consult the furnace model’s manual for specific diagnostic LED sequences and recommended part numbers. Rheem tech sheets often include microamp ranges, wiring diagrams, and error code definitions helpful for accurate troubleshooting.

FAQ

How Quickly Can A Dirty Flame Sensor Cause Code 13?

A flame sensor can cause intermittent issues within weeks depending on fuel quality and combustion conditions; most often Code 13 appears after months without maintenance.

Can A Furnace Restart Itself After Code 13?

Many systems attempt staged restarts but will lock out after multiple failed tries, requiring manual reset at the thermostat or control panel depending on model.

Is It Safe To Keep Resetting The Furnace?

Repeated resets without addressing the root cause can mask serious problems and increase risk. Use reset only to confirm the fault; then perform troubleshooting or call a technician.

Does Replacing The Flame Sensor Require Special Tools?

Replacing a flame sensor typically requires basic hand tools and careful handling; testing with a multimeter improves diagnostic accuracy. Follow manufacturer instructions for access and torque values.

Quick Diagnostic Checklist

• Confirm Power And Gas Supply: Thermostat, breakers, and gas valve position.
• Clean Flame Sensor: Remove and lightly sand sensor tip.
• Inspect Igniter: Look for cracks and test continuity.
• Check Burners & Venting: Clear debris and obstructions.
• Measure Flame Current: Verify microamp signal with a multimeter.
• Call A Pro If: Gas pressure, heat exchanger damage, or repeated lockouts.

This guide aims to empower homeowners with practical, safe steps to diagnose and address Rheem furnace Code 13 Flame Lost events while emphasizing the limits of DIY work and the importance of professional HVAC service for gas- and combustion-related repairs.