Carrier Furnace Error Code 33: Causes, Troubleshooting, and Fixes

The Carrier furnace error code 33 typically indicates a problem with the flame rollout or a flame sensor/ignition sequence fault depending on model series; this guide explains what Error Code 33 means, how to diagnose it safely, step-by-step troubleshooting, likely causes, parts to inspect, and when to call a professional. Follow safety precautions and turn off power before inspecting components.

Symptom	Likely Cause	Quick Action
Error Code 33 On Control Board	Ignition failure, flame sensor fault, blocked vent, or flame rollout	Reset power, inspect venting, clean flame sensor, check error history
System Lockout After Attempts	Persistent ignition attempts without flame	Call HVAC technician if resets fail
Odor Or Soot	Flame rollout or combustion issue	Shut down and call emergency service

What Error Code 33 Means For Carrier Furnaces

On many Carrier furnace control boards, Error Code 33 indicates the control did not detect a proper flame during the ignition sequence or it detected an abnormal flame condition such as flame rollout. Interpretation can vary by model, so referencing the specific furnace’s manual is essential.

Common interpretations include: failed pilot or hot surface ignition, weak or intermittent flame sensing, blocked combustion air or venting, or heat exchanger/rollout switch activation.

Why This Error Is Important

Flame and ignition faults pose safety and performance risks. A persistent Error Code 33 can cause repeated lockouts, reduced heating, higher energy use, or unsafe combustion byproducts entering living spaces. Addressing it promptly prevents damage and maintains safe operation.

Safety First: Precautions Before Troubleshooting

Always turn off power and gas before opening access panels. If there is a strong smell of gas, evacuate the home and call the gas company or emergency services immediately. Use a multimeter only if familiar with electrical safety, and avoid working on burners or gas lines without proper training.

Tools And Materials Commonly Needed

• Multimeter for continuity and voltage checks
• Screwdrivers and nut drivers to remove access panels
• Soft cloth and fine-grit sandpaper or emery cloth
• Shop vacuum or compressed air for cleaning
• Replacement flame sensor, ignitor, rollout switch, or control board if diagnosed
• Manufacturer’s furnace manual or model-specific wiring diagram

Step-By-Step Troubleshooting For Error Code 33

1. Record Error History And Reset

Check the control board LEDs or user interface to note the exact error code pattern and whether the system has locked out. Perform a single reset by turning off power at the breaker for 30 seconds and restoring power; observe whether the code returns.

2. Visual Inspection

Open the furnace access panel and inspect burners, pilot/ignitor area, vent connections, and the control board for obvious signs of damage, soot, corrosion, or loose wiring. Look for black soot, melted insulation, or disconnected harnesses.

3. Inspect Venting And Combustion Air

Ensure exhaust flue and intake air paths are clear of blockages, bird nests, debris, or ice. A blocked vent can cause flame disturbances or rollout and trigger Code 33.

4. Check The Flame Sensor

Remove the flame sensor rod and gently clean the metal rod with emery cloth or fine sandpaper to remove oxidation. Reinstall securely and restart. A dirty or corroded flame sensor is a frequent cause of flame detection failures.

5. Inspect The Ignitor

For hot surface ignitors, check for cracks or discoloration; a weak or failing ignitor may glow but not reach temperature, preventing reliable ignition. Replace if damaged. For spark or pilot ignition systems, verify proper spark and pilot function.

6. Test Rollout And Limit Switches

Check continuity on the rollout switch(s) and high-limit switch using a multimeter when the system is cold. A tripped or failed rollout switch indicates flame rollout or heat exchanger blockage and should not be bypassed.

7. Verify Gas Valve Operation

Confirm the gas valve is receiving the correct control voltage during the call for heat and that it opens. A malfunctioning gas valve or insufficient gas pressure can prevent flame establishment.

8. Examine Control Board And Wiring

Look for burned traces, swollen capacitors, or loose terminal connections on the control board. Verify wiring harnesses to the ignitor, flame sensor, and gas valve are secure and not damaged.

9. Test With A Known-Good Component

If possible, substitute a known-good flame sensor or ignitor to verify the diagnosis. This approach isolates whether the problem is component-specific or systemic.

Common Causes Of Carrier Error Code 33

• Dirty Or Faulty Flame Sensor: Most frequent cause; prevents flame detection.
• Weak Or Cracked Ignitor: Fails to ignite fuel reliably.
• Blocked Venting Or Combustion Air: Causes flame instability or rollout.
• Tripped Rollout Or Limit Switch: Indicates high heat or flame rollout conditions.
• Gas Supply Problems: Low pressure or closed valve prevents proper flame.
• Control Board Or Wiring Fault: Incorrect signals or open circuits disrupt ignition sequence.

Model Differences And Manuals

Carrier furnaces come in multiple series; Error Code 33 interpretation may vary. Always consult the furnace’s model-specific service manual or wiring diagram. The manual will provide exact LED blink codes, diagnostic flowcharts, and part numbers for replacement components.

When To Call A Professional Technician

If the error persists after basic checks—cleaning the flame sensor, verifying venting, and inspecting ignitor—and if any of these are found damaged, a qualified HVAC technician should be called. Do not bypass safety switches or continue operation when soot, odor, or flame rollout is present.

Call a professional immediately if there is any sign of carbon monoxide (CO) such as headaches, nausea, or if a CO detector alarms. Technicians can perform combustion analysis, gas pressure checks, and safe part replacement.

Parts Replacement Guide And Cost Estimates

Typical parts associated with Error Code 33 include the flame sensor, hot surface ignitor, rollout switch, and control board. Estimated retail part costs (US averages) are: flame sensor $25–$80, ignitor $40–$200, rollout switch $15–$50, control board $150–$500 plus labor.

Labor costs vary regionally; a standard diagnostic visit may cost $75–$150, with repairs often adding $150–$400 depending on complexity. Request a written estimate and ask for OEM or manufacturer-equivalent parts.

Preventive Maintenance To Avoid Error Code 33

• Schedule annual professional furnace inspection and cleaning before heating season.
• Replace filters regularly and ensure vents and returns are unobstructed.
• Keep outdoor vents clear of debris, snow, and vegetation.
• Inspect and clean the flame sensor during routine maintenance.
• Install and maintain carbon monoxide detectors within the home.

Troubleshooting Checklist

Action	Expected Result
Reset furnace power	Short-term clear if transient fault
Clean flame sensor	Resolves many detection faults
Inspect ignitor	Replace if cracked or weak
Check vents/intake	Clearing blockage stabilizes flame
Test rollout/limit switches	Replace if failed; do not bypass

Frequently Asked Questions About Error Code 33

Is It Safe To Run The Furnace With Error Code 33?

No. An Error Code 33 often reflects unsafe ignition or combustion conditions. Continuing operation risks repeated lockouts, damage, and potential safety hazards. Turn the system off and diagnose or call a technician.

How Long Does It Take To Fix?

Simple fixes like cleaning the flame sensor can take 15–30 minutes. Component replacements or diagnosing venting and combustion problems may take 1–3 hours depending on accessibility and parts availability.

Can Homeowners Replace The Flame Sensor Or Ignitor?

Handy homeowners comfortable with basic mechanical work and power shutoff can replace sensors or ignitors. Follow model-specific service instructions and ensure power and gas are off. If unsure, hire a licensed technician.

Will Resetting The Furnace Permanently Fix Code 33?

Resets may temporarily clear the code if caused by a transient issue, but recurring Error Code 33 indicates an underlying problem that requires inspection and repair.

Key Takeaways And Best Practices

Error Code 33 On Carrier furnaces commonly points to flame detection or ignition issues such as a dirty flame sensor, failing ignitor, blocked venting, or tripped rollout switch. Prioritize safety: shut power and gas off before service, do not bypass safety devices, and contact a qualified HVAC professional for combustion-related faults or persistent errors.

Document the model and serial number before calling for service, and keep a record of error patterns and troubleshooting steps to assist the technician in diagnosing the issue efficiently.

Resources And Next Steps

For model-specific diagnostics, refer to the Carrier furnace service manual or the control board diagnostic chart. Homeowners may also consult Carrier customer support or a local NATE-certified HVAC technician for inspection, combustion testing, and certified repairs.

If there is suspicion of carbon monoxide exposure, leave the home and call emergency services and the gas company immediately. Install and maintain CO detectors for ongoing safety monitoring.