The Rheem furnace code 13 (Flame Lost) indicates the furnace ignited but the control board lost the flame signal, causing a shutdown for safety. This article explains common causes, step-by-step diagnostics, practical troubleshooting, and when to call a professional. It focuses on safety and actionable solutions for U.S. homeowners and technicians.
| Issue | Likely Cause | Quick Action |
|---|---|---|
| Intermittent Flame Loss | Dirty flame sensor, weak ignition | Clean sensor, check ignition |
| Immediate Lockout | Gas valve or pressure, flame rectification failure | Check gas supply, inspect wiring |
| After Service Or Component Swap | Improper wiring or grounding | Verify wiring diagrams, re-tighten grounds |
What Rheem Furnace Code 13 Means
Rheem furnaces use diagnostic codes to indicate operational faults. Code 13—Flame Lost means the control board detected ignition but then stopped receiving the flame sensing signal during the trial for ignition or early run cycle. The board typically attempts a few retries before entering lockout for safety.
How Flame Sensing Works
Modern gas furnaces detect flame using the flame rectification principle. The metal flame sensor rod is placed in the burner flame and develops a small DC current when flame is present. The control board reads this microamp signal to confirm combustion. If the board does not see the signal, it assumes the flame is out and shuts down the gas valve.
Common Causes Of Code 13
Several factors can disrupt flame detection. Dirty Flame Sensor is the most frequent cause, followed by wiring issues, grounding problems, ignition instability, weak gas pressure, blocked burners, or a defective control board.
Dirty Or Corroded Flame Sensor
Residue, oxidation, or soot buildup on the sensor reduces the rectified current and prevents detection. This is common after prolonged operation or poor combustion.
Loose, Damaged, Or Disconnected Wiring
Wiring between the sensor and control board may be loose, corroded, or damaged, interrupting the signal. Poor connections at the board or sensor can be intermittent.
Poor Grounding Or Furnace Chassis Ground Issues
The millivolt signal returns through the furnace ground. A weak or missing ground path due to paint, rust, or loose screws can prevent flame rectification.
Weak Or Unstable Ignition
Ignitors that heat inconsistently or spark ignition systems with weak sparks can cause the flame to go out shortly after ignition. This results in loss of flame sensing.
Gas Supply Problems Or Low Gas Pressure
Inadequate gas flow due to closed valves, blocked lines, or low supply pressure can produce weak flames that fail flame sensing.
Burner Orifices Blocked, Incorrect Air-Fuel Mixture
Dirty burners or incorrect burner adjustment cause poor flame characteristics. A yellow or lifting flame may not provide a reliable sensing signal.
Defective Flame Sensor Or Control Board
Although less common, the flame sensor itself can fail electrically, or the control board input circuit can be defective. Both require replacement or advanced testing.
Safety Precautions Before Troubleshooting
Working on gas furnaces can be hazardous. Always shut off the electrical power at the breaker and turn off the gas supply before inspecting internal components. If a gas odor is present, leave the home and call the gas company immediately.
Tools And Materials Needed
Basic tools and supplies speed troubleshooting: a multimeter, insulated screwdrivers, fine-grit sandpaper or emery cloth, replacement flame sensor (if needed), wire brush, contact cleaner, socket set, and safety gloves. A combustion analyzer is useful for testing flame quality and gas pressure but not required for initial checks.
Step-By-Step Troubleshooting Guide
1. Check Error History And Reset
Note the error sequence on the control board LED or the furnace display. Reset the furnace by turning off power for 30 seconds and restoring power. If code 13 returns immediately or after a short run, proceed to diagnostics.
2. Inspect The Flame Sensor
Locate the flame sensor rod near the burners. Remove the sensor screw and gently pull out the rod. Inspect for carbon buildup or oxidation. Clean carefully with fine-grit sandpaper or emery cloth until the metal is bright. Reinstall securely and test.
3. Verify Wiring And Connections
Inspect the wire from the sensor to the control board for frays, loose terminals, or corrosion. Wiggle connectors while observing the furnace response. Tighten or replace connectors as needed.
4. Check Furnace Grounding
Ensure the furnace chassis has a solid ground connection to the home electrical system. Clean grounding screw areas and retighten. Use a multimeter to confirm continuity between the flame sensor and chassis ground.
5. Observe Flame Characteristics
Turn the furnace on and watch the burners during ignition. A healthy flame is steady and blue with minimal lifting. Yellow tips, fluttering, or flame impingement on the sensor can cause intermittent sensing. Address burner cleaning or adjustment if needed.
6. Test Flame Sensor Milliamps
With a multimeter set to microamps, measure the current generated by the flame sensor while burners are lit. Typical values range from 2 to 10 microamps depending on design. Values below recommended thresholds indicate poor sensing and likely need a replacement sensor or improved grounding.
7. Check Gas Valve And Pressure
Confirm the gas valve opens and supply pressure meets manufacturer specs. Low pressure or partial valve openings create weak flames that may not be sensed. Gas pressure testing should be performed by a qualified technician where required by local codes.
8. Inspect Ignitor And Ignition System
For hot surface ignitors, check for cracks or failure. For spark systems, verify consistent sparking and electrode gap. Weak ignition can cause flame failure after initial lighting.
9. Replace Components If Necessary
If cleaning and wiring checks do not resolve the fault, replace the flame sensor with a factory-recommended part. If the control board remains unresponsive to a verified flame signal, the board may be defective and require replacement by a qualified technician.
When To Call A Professional
Call a licensed HVAC technician when gas pressure testing, control board replacement, complex wiring troubleshooting, or persistent flame instability is required. Professionals can safely test gas valves, measure flame rectification precisely, and comply with local codes. Persistent or recurring Code 13 after basic troubleshooting indicates deeper system issues.
Preventive Maintenance To Avoid Code 13
Regular maintenance reduces the risk of flame loss. Recommended tasks include annual furnace tune-ups, flame sensor cleaning, burner inspection and cleaning, verifying proper ventilation, and keeping the area clear of debris. Replace air filters regularly to reduce dust buildup that can affect combustion.
Common Misdiagnoses And Pitfalls
Assuming a new flame sensor will always fix Code 13 is a common mistake. Failure to address grounding, burner condition, or wiring often leads to repeat problems. Also, replacing the control board without verifying the flame signal wastes time and money.
Cost Estimates For Repairs
Costs vary by part and labor. Cleaning and basic service may cost $75–$200 if performed by a technician. A flame sensor replacement part typically costs $20–$80 plus labor. Control board replacements range widely, often $300–$700 installed. Gas valve or pressure-related repairs can be more expensive depending on complexity.
Documentation And Manufacturer Resources
Consult the specific Rheem furnace model’s installation and service manual for wiring diagrams and flame sensor specifications. Manufacturer bulletins and accessory manuals provide test values and troubleshooting flowcharts. Keeping model and serial numbers handy speeds accurate diagnosis and part ordering.
Key Takeaways And Best Practices
- Clean The Flame Sensor First: It’s the most common and simplest fix.
- Verify Wiring And Grounding: A secure ground is essential for flame rectification.
- Observe Flame Quality: Proper combustion prevents sensing problems.
- Use Correct Replacement Parts: OEM sensors and boards reduce compatibility issues.
- Call A Pro For Gas-Related Issues: Safety and code compliance are critical.
Additional Resources And Links
Refer to the Rheem technical support page and the furnace model manual for official guidance. For safety concerns or suspected gas leaks, contact local utilities or licensed HVAC professionals immediately.
Note: This article provides general guidance. Specific furnace models and local codes may require different procedures. When in doubt, consult a licensed technician.