Top 10 Lennox Furnace Problems

Step 1: Thermostat Problem

Check to make sure the thermostat is calling for heat, if the thermostat is battery operated check to see if the batteries need to be replaced.

Step 2: Main Circuit breaker Problem

Next, make sure that there is power going to the furnace. Check to see if the furnace breaker is turned off or moved in the middle, if it is turned off you can simply turn it back ON and if it’s moved in the middle then you will turn it to fully to the OFF position and then turn it back to ON position. This is how you can reset the breaker.

Note: if the breaker is tripped that means there is a possibility of a short circuit in the furnace or electric wire, you then should call an HVAC contractor immediately.

Step 3: Furnace Error Codes

Remove the front cover of the furnace. You will see the top and bottom compartments of the furnace. On the bottom compartment, you will find a see-through round glass. This see-through glass is to check the furnace fault codes. You can check how many times the LED either red or green or combinations of both lights are flashing. Note the flashing sequence and match it with the LED error codes given on the back of the furnace cover or on the user manual. It depends on what brand of furnace you have will have its own troubleshooting flow chart. You can also find your Furnace error codes at the bottom of this page.

Lennox Furnace Flash Codes

LED #1 (Red) LED #2 (Green) DESCRIPTION





Power on – Normal Operation.

It also signaled during cooling and continuous fan.





Normal Operation – singled when heating demand initiated at the thermostat.
SLOW FLASH ON Primary or secondary limit switch open. The limit must be close Within 3 minutes or the unit goes into 1 minute Watchgurad.

Pressure prove switch open.

OR: Blocked inlet/ exhaust vent;

OR: pressure switch closed prior to activation of combustion air inducer.




Watch guard – burner failed to ignite or lost flam 5 times during single heating demand
SLOW FLASH OFF Flame sensed without gas valve energized.
ON SLOW FLASH Rollout switch open. OR: 12-pin connector improperly attached.







Circuit board failure or control wired incorrectly.
FAST FLASH SLOW FLASH Main power polarity reversed. Switch line and neutral.
SLOW FLASH FAST FLASH Low flame signal. Measures below 1.5 microamps. Replace flame sense rod

Improper main ground.

OR: Line voltage below 90 volts.

NOTE – Slow flash rate equals 1 Hz (one flash per second). Fast flash rate equals 3 Hz (three flashes per second). Minimum flame sense current = 0.5 microAmps.

Step 4: Inducer Motor Problem

Once the thermostat calls for heat, the inducer motor comes on to make sure there is no restriction in the exhaust and intake vent. Inducer motors exhaust the flue gases out of your home when the furnace is working, if the inducer motor stops working your furnace also stops working.

The Inducer motor is located on the top compartment of the furnace.

A simple way to test the inducer motor is by touching it, if it feels too hot that means it’s burnt. Be careful when touching the motor, sometimes it becomes too hot and can burn your fingers.

Step 5: Furnace Vents 

On High-efficiency furnaces check to see there is no restriction in front of exhaust and intake vent pipes. Sometimes snow piles up and blocks the exhaust or intake vent pipes, which causes the pressure switch to open. A pressure switch that senses negative pressure in the venting turns off the furnace. The furnace pressure switch ensures there is no blockage in vent pipes.

Step 6Igniter Problem:

  • Standing Continuous Pilot

Pilot light out is the most common problem among older furnaces. These furnaces don’t have an Inducer motor. On a call for heat, the gas valve opens and passes the gas to the burners where a small pilot flame that stays ON all the time lights the main burners.

  • Spark Igniter 

Some high-efficiency furnaces have spark igniters to light the burners. On the call for heat once the inducer motor ensures safe operation then the circuit board sends a signal to spark igniter. It stops sparking after lightening the burners….

  • Hot Surface Igniter (HSI)

Most high-efficiency furnaces have HSI. On the call for heat once the inducer motor ensures safe operation then the circuit board sends a signal to HSI. It glows and lights the burners and stops glowing after lighting the main burners.

(Note) Depending on which furnace you have, if you can see the pilot light, HSI glow, or hear a spark but the burners don’t come on then it’s a furnace Gas Valve, Circuit Board, or Gas related issue.

Step 7: Flame Sensors Problem

When the burners come ON a flame sensor proves the presence of a flame, without this flame sensor furnace would not know when a flame is present. Furnace’s short cycling is also one of the reasons for flame sensor failure. The circuit consists of a flame sensing rod at the opposite end of the burner assembly from the furnace gas manifold connection. The flame sensing rod is positioned in front of the last burner.

Causes of low or no flame current:

  • Dirty flame rod
  • Cracked ceramic insulator on flame sensing rod
  • Cracked or broken flame sensing rod wire
  • Poor furnace ground
  • Reverse polarity of 115-volt power
  • Low gas supply

Step 8: Low Gas Pressure:

Normal L.P Gas pressure 10” to 12.5” W.C. Normal Natural gas pressure is 7” to 11” W.C. Only a certified gas technician can check the gas pressures by using a manometer. Check to make sure your gas supply is in the on position parallel to your gas supply line

Step 9: Clogged Drain

All high-efficiency furnaces are condensing also known as Category 4 furnaces. All the condensation collects in a collector box where the water exits through a drainpipe to the house drain. Check to see if the drain is clogged. Vacuum it with a shop vacuum to remove any blockage. If the drain gets clogged, it stops the furnace. It can also cause a short circuit of the circuit board, blower motor, poor air quality, and rusting of the furnace. This is why regular furnace maintenance does more than just keep them running.

Step 10:Furnace Filter

The furnace filter is also a major cause of furnace Blowing cold air.

The furnace filter is generally on the return duct right before the furnace.  A dirty filter causes blockage of airflow. This results in a furnace overheating and shutting off too quickly before your house gets heated up.

It is recommended to replace the furnace filters every 2 to 3 months.