The Lennox furnace primary/secondary limit switch open condition indicates a safety device has stopped normal operation, requiring diagnosis to restore heating safely and efficiently.
| Symptom | Possible Cause | Quick Action |
|---|---|---|
| Furnace Won’t Heat | Open Limit Switch | Check airflow and filters |
| Short Cycling | Blocked Vent Or Overheat | Inspect flue and blower |
| Blower Runs Continuously | Stuck Switch Or Control Fault | Test switch continuity |
How Lennox Primary And Secondary Limit Switches Work
Primary and secondary limit switches are temperature-activated safety controls that prevent overheating and unsafe furnace operation. The primary limit typically monitors the heat exchanger or plenum temperature, opening when temperatures exceed safe thresholds. The secondary limit serves as a backup or monitors a different location for redundancy. When either switch opens, the furnace may lock out gas valve operation or trigger the blower to run until temperatures drop.
Common Causes Of A Limit Switch Open Condition
Several conditions can cause a limit switch to open. Poor airflow is the most frequent culprit, often due to clogged filters, closed vents, or a failing blower motor. Blocked exhaust or intake vents and restrictive ducts can create hot spots. Faulty switches, wiring problems, and control board faults also produce open readings. Less commonly, a cracked heat exchanger or combustion issue leads to abnormal temperatures that trip the limit.
Symptoms That Indicate The Limit Switch Is Open
- Furnace fails to ignite or holds in lockout state after a call for heat.
- Furnace runs briefly then shuts down (short cycling).
- Blower continues to run after burners shut off.
- Service indicator codes on Lennox control board or LED diagnostics.
- Unusual attic or duct heat, high supply air temperatures.
Diagnostic Steps To Confirm An Open Limit Switch
Before testing electrical components, ensure power is off and follow safety protocols. Visual and basic checks can identify simple causes quickly.
1. Check Air Filters And Vents
Replace dirty filters and ensure all supply and return vents are open and unobstructed. Restricted airflow commonly causes overheating and limit trips.
2. Inspect Blower And Motor
Confirm the blower motor runs smoothly and the wheel is clean. Worn motor bearings or a damaged wheel reduce airflow and raise temperatures.
3. Examine Venting And Combustion Air
Verify the furnace flue, vent pipes and outside intake are clear of debris, nests, or snow. Blocked vents lead to heat buildup and limit activation.
4. Read Furnace Diagnostic Codes
Many Lennox furnaces use LED flash codes or a control board display. Cross-reference the code with the model manual to confirm a limit-related fault.
5. Test The Limit Switch With A Multimeter
With power off, locate the primary and secondary limit switches on the furnace plenum or near the heat exchanger. Remove leads and test for continuity at ambient temperature. A normally closed (NC) limit should show continuity; an open reading indicates an open switch. Document readings for both switches and compare to expected values.
How To Safely Bypass Or Reset A Limit Switch
Bypassing limit switches is not recommended except for temporary diagnostics by a qualified technician. Removing safety devices can create unsafe conditions. If a reset button exists on the limit switch, a single manual reset may restore operation, but repeated trips indicate an underlying issue that must be addressed.
When A Limit Switch Is Faulty Versus Secondary Causes
A defective limit switch will fail continuity at normal temperatures or display physical damage such as burned contacts. If switches test good, focus on airflow, venting, heat exchanger condition, and control board issues. Recurrent trips after cleaning and repairs suggest a defective switch, wiring, or control board failure.
Replacing A Lennox Primary Or Secondary Limit Switch
Replacement should follow OEM specifications for temperature set points and terminal configurations. Use genuine Lennox parts or equivalent components with identical ratings. Steps include turning off power and gas, labeling wires, removing the old sensor, installing the new switch securely, reconnecting wiring, and verifying operation through diagnostics.
Tools And Parts Required
- Multimeter
- Screwdrivers and nut drivers
- Replacement limit switch (model-specific)
- Wire labels or tape
- Manufacturer’s wiring diagram
Estimated Labor And Parts Cost
Parts for a limit switch typically range from $20 to $80. Professional labor varies by region and complexity, commonly $100 to $300. If diagnosis reveals additional issues like a failing blower or blocked venting, costs will rise accordingly.
Preventive Maintenance To Avoid Limit Switch Opens
Routine furnace maintenance greatly reduces the risk of limit switch trips. Recommended actions include replacing filters every 1–3 months, annual professional tune-ups, cleaning blower assemblies, inspecting venting, and ensuring adequate return air pathways. Adequate combustion air and proper duct design prevent hot spots and improve system longevity.
Safety Considerations And When To Call A Professional
Limit switches are safety devices tied to combustion and overheating risks. If a limit opens repeatedly, or if there are smells of gas, unusual noises, or visible heat exchanger concerns, shut off the furnace and contact a licensed HVAC technician. Working on gas components and combustion systems requires certification and appropriate safety testing.
Common Lennox Models And Known Limit Switch Locations
Limit switch placement varies by model. For many Lennox furnaces, primary limits are mounted on the plenum or near the heat exchanger outlet. Secondary limits may be located on the opposite side of the plenum or near the blower compartment. Refer to the model-specific schematic or manual for exact locations.
Interpreting Control Board Codes And Logs
Lennox control boards often store fault history. Technicians should record fault frequencies and times to correlate with environmental conditions or operational cycles. A pattern of trips during high-demand periods points to airflow or sizing issues, while random trips may indicate electrical instability or intermittent sensor failures.
Cost-Benefit Of DIY Versus Professional Service
Homeowners can perform basic checks like replacing filters and clearing vents. Multimeter testing of limits is possible for those comfortable with electrical safety. However, accurate diagnosis of recurrent limit trips, combustion evaluation, and heat exchanger inspection require professional equipment and expertise. Improper repairs risk safety and liability.
Frequently Asked Questions
What Does “Limit Switch Open” Mean On A Lennox Furnace?
It means a safety thermostat has opened its electrical contacts due to high temperature or a detected fault, preventing normal burner operation until temperatures normalize or the issue is resolved.
Can A Dirty Filter Cause A Limit Switch To Open?
Yes. A dirty filter restricts airflow, raises internal temperatures, and commonly causes the primary limit switch to open.
How Long Can The Furnace Sit After A Limit Trip?
Most furnaces will attempt resets or require manual reset depending on the model. Allow the unit to cool and follow the manufacturer’s reset procedure; repeated trips require professional inspection.
Is It Safe To Replace The Limit Switch Without Shutting Off Gas?
No. Shut off power and gas before replacing any safety control to prevent ignition, injury, or equipment damage.
Resources And Further Reading
Consult the Lennox owner’s manual and the furnace’s wiring diagram for model-specific diagnostics. Manufacturer technical bulletins and HVAC trade resources provide advanced troubleshooting. Local licensed HVAC providers can perform comprehensive safety checks, combustion analysis, and certified repairs.
For safe, long-term operation, prioritize regular maintenance, prompt attention to diagnostic codes, and replacement of failed safety components with OEM-specified parts.