If you are seeing this code, you are likely experiencing a furnace that refuses to ignite, or one that starts for a few minutes before shutting down while the blower motor continues to run indefinitely, blowing cold air through your vents. You might also hear a repetitive clicking sound as the system attempts to reset. While this error sounds intimidating, it is often caused by simple airflow restrictions. Don’t worry—this guide will walk you through the diagnostic process to get your home warm again safely.
Quick Repair Specs
| Difficulty | Intermediate (Requires basic electrical testing) |
| Estimated Time | 30 – 60 Minutes |
| Tools Needed |
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| Estimated Cost | $15 (Filter) to $65 (Limit Switch) |
Symptoms of Lennox Error 9
When the high limit switch opens, the furnace enters a “watchguard” or lockout mode to protect the structural integrity of the heat exchanger. You will notice the following physical signs:
🛠️ Warning: Check Manual First
For your safety and to avoid voiding the warranty, please check the official docs.
- The Diagnostic LED Flash: Behind the small sight glass on the lower furnace door, you will see a red LED blinking 9 times in a row, pausing, and then repeating.
- Continuous Blower Operation: The large blower fan may run non-stop. This is a safety feature designed to cool down the overheated heat exchanger as quickly as possible.
- Cold Air from Vents: Because the gas valve is disabled while the limit switch is “open,” the burners will not stay lit, meaning the air circulating through your home will feel cold or lukewarm.
- Short Cycling: The furnace may attempt to start, ignite for 30–90 seconds, and then abruptly shut down before the house reaches the desired temperature.
Detailed Diagnosis: Root Causes
The High Limit Switch is a bimetallic disc that reacts to temperature. If the air passing over the heat exchanger isn’t moving fast enough to carry the heat away, the switch “trips.” Understanding why this happens is key to a permanent fix.
- Restricted Airflow (Most Common): A heavily soiled air filter acts like a wall. When the blower cannot pull enough air through the filter, the heat exchanger’s temperature skyrockets. Over time, this “heat soaking” can also weaken the switch’s internal spring, causing it to trip at lower-than-rated temperatures.
- Blower Motor or Capacitor Failure: If the blower motor is spinning too slowly—often due to a failing “run capacitor” or a buildup of grime on the squirrel cage blades—it won’t move enough CFM (Cubic Feet per Minute) to cool the system. This leads to a slow temperature creep until the limit is reached.
- Obstructed Return or Supply Vents: Homeowners often close registers in unused rooms or cover return air grilles with furniture. This creates static pressure imbalances. The furnace is essentially “suffocating,” causing the internal plenum temperature to exceed the safety limit (usually around 140°F–170°F depending on the model).
- Faulty Limit Switch (Component Wear): After years of voltage spikes or repetitive “tripping” due to dirty filters, the switch can physically fail in the “open” position. Even when the furnace is stone cold, the switch remains electrically disconnected, preventing the furnace from ever starting.
Comprehensive Repair Guide
Follow these steps in order. WARNING: Always turn off the electrical power at the breaker or the service switch located on the side of the furnace before removing any panels.
- The Filter Inspection:
Start by removing your furnace filter. If it is grey, dusty, or caked in pet hair, this is likely your culprit. A simple way to test if the filter is the cause is to temporarily run the furnace without the filter for 10 minutes (only for testing). If the Error 9 clears and the furnace stays lit, simply replace the filter with a new one and the job is done. Avoid high-MERV “allergen” filters if your furnace is older, as they can sometimes be too restrictive. - Checking for Ductwork Obstructions:
Walk through your home and ensure that at least 80% of your supply registers are fully open. Check that large area rugs or couches are not covering the return air grilles. If the furnace cannot “breathe” in, it cannot blow heat out, leading directly to a limit trip. - Accessing the High Limit Switch:
Unscrew the upper burner compartment door using your nut driver. Locate the high limit switch—it is typically mounted on the firewall directly above the burners or on the blower housing. It is a small, usually circular or rectangular component with two wires (often yellow or orange) attached to it. - Testing Continuity with a Multimeter:
With the power still OFF, pull the two wire connectors off the switch terminals. Set your multimeter to the “Ohms” or “Continuity” (beep) setting. Place one probe on each terminal of the switch. If the furnace is cool, the meter should show 0 Ohms (or beep). If the meter shows “OL” or “1” (Infinite Resistance) while the furnace is cold, the switch is defective and must be replaced. - Installing the Replacement Switch:
If the switch is failed, remove the two mounting screws. Ensure your replacement switch has the exact same temperature rating (printed on the side of the switch, e.g., “L170-20F”). Screw the new switch into place, reattach the wires, and replace the furnace doors. - Power Restoration and Reset:
Turn the power back on. The furnace control board will perform a self-test. If the “9 red flashes” have disappeared and you see a slow, steady blink (normal operation), your repair was successful.
How to Prevent Error 9 Flashes Red
To ensure your Lennox furnace remains reliable and to prevent the premature wear of the safety components, follow these maintenance protocols:
- Adopt a “Filter Schedule”: Set a calendar reminder to check your filter every 30 days. If you have pets or live in a dusty environment, you may need to replace it monthly. A clean filter is the single best way to protect your heat exchanger.
- Clean the Blower Assembly: Once a year, use a vacuum with a brush attachment to clean the blower motor and the “fins” of the fan wheel. Dust buildup on these fins reduces airflow efficiency, which forces the furnace to run hotter.
- Annual Professional Inspection: Have an HVAC technician perform a static pressure test. This measures how hard the fan has to work to push air through your ducts. If the static pressure is too high, they can adjust the motor speed taps to increase airflow and keep the system below the high-limit threshold.
FAQ
Frequently Asked Questions
Q: Can I bypass the high limit switch just to get through the night?
A: Absolutely not. Never jumper or bypass a high limit switch. This switch is the only thing preventing your heat exchanger from cracking or catching fire if the blower fails. Bypassing it creates a significant risk of carbon monoxide poisoning or a house fire.
Q: Why does the fan keep running if the furnace is off?
A: When the control board detects an open limit (Error 9), it enters a safety loop. It assumes the furnace is dangerously hot, so it energizes the blower motor to move air and cool the metal down. It will stay in this mode until the switch closes or power is cycled.
Q: My filter is clean, but I still get Error 9. What else could it be?
A: If the filter is clean and the switch is functional, you may have an internal blockage such as a dirty evaporator coil (if you have central air) or a failing blower motor capacitor. These issues require more advanced tools to diagnose and may require a professional.