How to Open High Limit Switch on Goodman Furnace Safely

The high limit switch is a critical safety component in a Goodman furnace that shuts the burner off if the heat exchanger becomes too hot. This guide explains when and how to open or test the high limit switch, common causes of trips, troubleshooting steps, and safety precautions. It focuses on practical, accurate, and actionable information for U.S. homeowners and technicians.

Task	Time	Difficulty
Visual inspection	5–10 minutes	Easy
Manual reset/test (non-resettable models require replacement)	10–30 minutes	Moderate
Replace high limit switch	30–60 minutes	Moderate–Advanced

What Is The High Limit Switch And Why It Matters

The high limit switch monitors heat exchanger temperature and interrupts the furnace burner if temperatures exceed safe limits. Goodman furnaces use this switch to prevent overheating that can damage components or create a safety hazard. If the high limit opens (trips), the furnace will stop heating until the condition is resolved.

High limit switches can be either manual-reset or auto-reset. Auto-reset models close again when temperature falls; manual-reset models require a button press or replacement to restore operation.

Common Symptoms When The High Limit Switch Opens

• Furnace runs briefly and then shuts off without producing adequate heat.
• Blower continues to run while burners are off.
• Frequent cycling on and off, especially under high heat demand.
• Error codes on Goodman control boards indicating high limit or overheating.
• Visible dirt or restricted vents causing reduced airflow.

Safety First: Precautions Before Opening Or Testing

Working on a furnace involves electrical and combustion hazards. Always turn off power to the furnace at the service switch or breaker and shut off the gas supply when performing tests or replacing parts.

Wear safety glasses and insulated gloves. If the furnace is under warranty or if the homeowner is uncomfortable with HVAC work, contact a licensed HVAC technician. Improper handling can lead to fire, carbon monoxide, or equipment damage.

Tools And Materials Needed

• Multimeter (with continuity and temperature functions if possible)
• Screwdrivers (Phillips and flathead)
• Nut drivers or socket set
• Needle-nose pliers
• Replacement high limit switch (specific to Goodman model)
• Small mirror and flashlight for inspection
• Manufacturer’s wiring diagram or manual

Locate The High Limit Switch On Goodman Furnaces

The high limit switch is typically mounted on the furnace plenum or near the heat exchanger access panel. On Goodman units it often appears as a small round or rectangular device with two or three wires connected.

Consult the furnace wiring diagram (usually on the inside of the service panel) to confirm the component. The limit switch is commonly labeled “LIM,” “HL,” or “High Limit” on schematics.

Step-By-Step: How To Open Or Test The High Limit Switch

1. Power Off And Prepare

Turn off the furnace power at the service switch and circuit breaker. Shut off the gas if performing work on the gas valve or burners. Wait a few minutes for components to cool and to ensure capacitors discharge.

2. Access The Switch

Remove the furnace access panel to locate the high limit switch. Use a flashlight to identify the switch and note its connections. Take a photo for reference before disconnecting wires.

3. Visual Inspection

Inspect the switch and surrounding area for dirt, corrosion, melted wires, or signs of overheating. Check the air filter, return vents, and blower wheel for restrictions that cause overheating.

4. Test For Continuity With A Multimeter

Set the multimeter to continuity or ohms. Disconnect wires from the limit switch terminals. Place probes on the switch terminals. A cold, normally closed high limit should show continuity. A tripped limit may show open (no continuity).

5. Test Under Temperature Or Manually Reset

If the furnace has a manual reset button on the limit switch, press it after cooling. For thermal diagnosis, use a temperature probe or heat source to confirm switch operation per OEM temperature ratings. Only perform temperature tests if qualified.

6. Check Control Board And Wiring

Trace wiring from the limit switch to the control board. Look for loose connectors or frayed insulation. A faulty thermostat, control board, or short can mimic a high limit trip. Verify the control board error codes for high limit faults.

When The High Limit Switch Is Tripped: Troubleshooting Causes

• Restricted Airflow: Dirty filters, blocked return vents, closed dampers, or plugged coils reduce airflow and cause overheating.
• Blower Problems: Weak motor, failing capacitor, or dirty/blade imbalance reduces circulation.
• Improper Duct Design: Undersized return or supply ducts increase static pressure and reduce flow.
• Flame Or Burner Issues: Incomplete combustion or delayed ignition can cause heat spikes in the exchanger.
• Faulty Switch: Age, corrosion, or mechanical failure can cause nuisance opens.
• Control Board Faults: Incorrect wiring, a failed relay, or sensor faults can create false trips.

How To Reset A Tripped High Limit Switch

For auto-reset models the switch will close automatically once temperatures drop below the threshold; restarting the furnace normally restores operation. For manual-reset models press the reset button only after the furnace has cooled and the cause is corrected.

If the switch does not reset or trips again immediately, do not repeatedly reset. That suggests an unresolved overheating issue that requires professional diagnosis.

Replacing The High Limit Switch On A Goodman Furnace

Replacing a failed high limit switch is straightforward for a competent DIYer but requires attention to match OEM specifications. Always buy an exact replacement part that matches the original switch’s temperature rating and physical mount.

1. Turn power and gas off.
2. Document wire locations and remove connectors.
3. Unscrew the old switch and install the new one in the same location.
4. Reconnect wiring per the photo or wiring diagram.
5. Restore power and test operation, observing startup cycles and control board indicators.

Tips To Prevent High Limit Trips

• Change air filters every 1–3 months depending on use and filter type.
• Schedule annual HVAC maintenance to clean the blower, burners, and heat exchanger.
• Keep return registers and supply vents unobstructed and balanced.
• Ensure proper thermostat settings and avoid placing heat-producing devices near the thermostat.
• Address unusual odors, sounds, or performance changes promptly.

When To Call A Professional HVAC Technician

If trips recur after basic troubleshooting, if burners or heat exchanger show damage, or if gas controls or electrical components are involved, contact a licensed HVAC technician. Repeated trips can indicate safety-critical issues such as cracked heat exchangers or gas valve faults that require professional inspection.

Technicians can perform detailed diagnostics including combustion analysis, airflow measurements, blower bench tests, and control board evaluation.

Interpreting Goodman Furnace Error Codes Related To High Limit

Goodman control boards often indicate limit faults via LED blink sequences or diagnostic codes on an attached display. Consult the furnace’s service manual for specific blink-code meanings.

Common indicators include sequences that point to high limit open, flame rollout, or pressure switch failures that can mimic limit trips. Record the code and share it with a technician for faster diagnosis.

Cost Considerations

Replacement high limit switches typically cost between $20 and $80 for the part alone, depending on OEM vs aftermarket. Labor for professional replacement ranges widely, often $75–$200 depending on service call rates and complexity.

Addressing root causes like airflow improvement or blower repair can increase total costs but prevents repeat failures and protects the furnace lifespan.

Key Takeaways And Best Practices

• Safety First: Always shut off power and gas before working on the furnace.
• Diagnose Root Causes: Fix airflow, blower, and duct issues to prevent repeat trips.
• Document Wiring: Photograph connections before disassembly to ensure correct reassembly.
• Use OEM Parts: Match temperature rating and mounting for replacements on Goodman units.
• Call Professionals: For combustion, control board, or repeated safety trips, contact a licensed technician.

Problem	Likely Fix
Limit trips after short runtimes	Replace dirty filter, clean blower, check vents
Limit open with no airflow	Inspect blower motor/capacitor and belt (if applicable)
Limit resets then trips immediately	Check duct restrictions, thermostat, or cracked heat exchanger

For complex diagnostics, including heat exchanger inspections and combustion analysis, working with an HVAC professional ensures safe, code-compliant repairs. Proper maintenance reduces the likelihood of high limit issues and extends furnace service life.