The blinking lights on a Lennox furnace control board are diagnostic codes. On most older Lennox furnaces (G40, G43, G50, G60, G61 and 80MGF series), two LEDs, usually one red and one green, flash in combinations. Both lights flashing slowly together means the furnace has power and is waiting for the thermostat. Both flashing quickly means it is heating normally. Any other pattern points to a specific fault, such as an overheating limit switch, a pressure switch problem or a failed ignition. Many newer Lennox furnaces use a single LED that flashes a set number of times instead. Always match the pattern to the code chart printed on the inside of your furnace door, because codes vary by model.
It is the first truly cold night of the season in Highlands Ranch. The thermostat says HEAT, the vents are blowing cool air, and when you pop the panel off the furnace there is a little light blinking at you. That light is actually good news. Your Lennox furnace is telling you exactly where it got stuck.
PS Cool Heat has been servicing furnaces across the Denver Metro area since 2015, and as a Lennox Network Dealer our technicians read these codes every day. Here is how to read yours, what you can safely check on your own, and when it is time to call for furnace repair.
Where to find the light on your Lennox furnace
The control board sits in the lower section of the furnace, behind the blower door. Most Lennox furnaces have a small clear window in that door so you can see the LED without removing anything.
How to read your code
- Do not shut off power first. Turning the furnace off clears the stored code, and you lose the clue.
- Watch the light for at least three full cycles. Note the color, whether it is flashing fast or slow, and if it flashes a set number of times before pausing.
- Take a short video on your phone. It makes counting easier and helps our technician diagnose faster.
- Find the code chart. Lennox prints a diagnostic label on the inside of the blower door. That chart is the final word for your exact model.
- Write down the model number from the rating plate inside the cabinet. It tells a technician which board and code set you have.
Lennox furnace red and green light codes (two-LED boards)
Most Lennox furnaces from the G40, G43, G50, G60, G61 and 80MGF families use two LEDs. On most of these boards the first LED (DS1) is red and the second (DS2) is green. If both of yours are the same color, go by the DS1 and DS2 labels on the board.
| Red (DS1) | Green (DS2) | What it means | Common causes | What to do |
|---|---|---|---|---|
| Slow flash | Slow flash (together) | Normal. Power is on, no call for heat. | Thermostat is not calling for heat. | Check that the thermostat is on HEAT and set above room temperature. |
| Fast flash | Fast flash (together) | Normal. Furnace is responding to a call for heat. | None. | Nothing. Give it a few minutes to light. |
| Off | Off | No power to the board. | Tripped breaker, service switch off, blown board fuse, door not seated. | Check the breaker and the switch near the furnace. If a fuse blew, call us. |
| Slow flash | Solid | High limit switch open (furnace is overheating). | Dirty filter, closed or blocked vents, blower problem. | Replace the filter and open vents. If it returns, call. |
| Off | Slow flash | Pressure switch did not close. | Blocked intake or exhaust pipe, inducer motor, cracked hose, condensate backup. | Check outdoor vent pipes for snow or debris. Otherwise call. |
| Slow flash | Slow flash (alternating) | Ignition lockout. Burners failed to light. | Dirty flame sensor, worn ignitor, gas supply or gas valve issue. | One reset is fine. If it returns, call. |
| Slow flash | Fast flash | Low flame signal. | Dirty flame sensor, low gas pressure. | Schedule service. Often fixed during a tune-up. |
| Solid | Slow flash | Flame rollout switch open. | Blocked or damaged heat exchanger, venting problem. | Safety code. Do not keep resetting. Call right away. |
| Slow flash | Off | Flame sensed with the gas valve off. | Gas valve or control board fault. | Safety code. Turn the furnace off and call. |
| Fast flash | Slow flash | Line voltage polarity reversed. | Wiring problem. | Call a technician. |
| Fast flash | Fast flash (alternating) | Improper ground or voltage. | Wiring, fuse or ignitor issue. | Call a technician. |
| Solid | Solid | Control board self-check failure. | Board or wiring fault. | Call a technician. |
Lennox furnace red light flashing 3, 5, 7 or more times (single-LED boards)
Many Lennox ML and EL series furnaces use one LED that flashes a number of times, pauses, then repeats. A steady light that is not flashing usually means normal operation. Count the flashes between pauses and compare them to the chart below and to the label on your door.
| Flashes | What it typically means | Most likely causes |
|---|---|---|
| 1 | Flame sensed while the gas valve is off | Gas valve or control board fault. Turn off and call. |
| 2 | Pressure switch closed while the inducer is off | Stuck pressure switch or board fault |
| 3 | Pressure switch open while the inducer is running | Blocked vent or intake, inducer motor, cracked or wet pressure switch hose, condensate drain backup |
| 4 | Primary limit switch open (overheating) | Dirty filter, closed vents, blower motor or capacitor |
| 5 | Flame rollout switch open | Blocked flue or heat exchanger, inducer problem. Safety code. Call. |
| 6 | Pressure switch lockout after repeated tries | Same causes as 3 flashes, but persistent |
| 7 | Ignition lockout, burners failed to light | Ignitor, flame sensor, gas supply, gas valve |
| 8 | Lockout after losing flame too many times | Dirty or failing flame sensor, low gas pressure |
| 9 | Line voltage polarity reversed | Wiring problem |
| 10 or more | Varies by board | Check the label on your furnace door, or call us with the model number |
Some newer Lennox furnaces, especially communicating systems paired with a Lennox smart thermostat like the S40, show alphanumeric codes on a small digital display or right on the thermostat screen. Those codes are model-specific, so a quick photo and your model number are the fastest path to an answer.
What does a flashing green light on a Lennox furnace mean?
Context is everything. A green light flashing slowly in sync with the red light is normal standby. A green light flashing slowly while the red light is off points to the pressure switch, which usually means something is restricting the venting. A green light flashing fast alongside a slow red light means the furnace is lighting, but the flame signal is weak, which is most often a dirty flame sensor.
What you can safely check yourself
Before you call, these five checks fix a surprising number of furnace codes, and none of them involve opening the gas system.
- Thermostat: set to HEAT, set a few degrees above room temperature, fresh batteries.
- Air filter: a clogged filter is the number one cause of limit switch codes. Between spring dust, wildfire smoke and pets, Front Range filters load up faster than most people expect.
- Power: check the breaker and the light-switch-style service switch near the furnace, and make sure the blower door is pushed in all the way. A loose door trips a safety switch.
- Outdoor vent pipes: high-efficiency furnaces vent through two white PVC pipes on an outside wall or the roof. Drifting snow during Colorado storms can block them and trigger pressure switch codes. Clear them gently by hand.
- One reset: turn the furnace off at the service switch for 30 seconds, then back on. If the same code comes back, stop resetting and call.
What not to do: do not repeatedly reset a rollout or flame-sensed code, do not bypass any safety switch, and do not attempt to adjust or replace the gas valve. Those codes exist to protect your family.
Safety first: If you smell rotten eggs or gas, do not flip switches or use your phone inside. Leave the house, then call 911 or your gas utility’s emergency line (Xcel Energy for most Denver Metro homes). If your carbon monoxide alarm goes off, get everyone outside and call 911.
Why Colorado furnaces throw certain codes more often
- Altitude: Front Range homes sit well above 5,000 feet. Thinner air changes combustion, so furnaces need to be set up for altitude during installation. A furnace that was not configured correctly is more likely to throw pressure switch and flame codes.
- Snow and wind: drifting snow blocks low PVC vent terminations, which shows up as pressure switch codes.
- The first fire of fall: after sitting idle all summer, dust settles on flame sensors and burners. October is when we see a spike in ignition and low flame codes, which is exactly why annual furnace maintenance before the first cold snap pays off.
When to call PS Cool Heat
- The same code returns after one reset
- You see a rollout, flame-sensed or polarity code
- The burners light and then shut off within seconds
- You smell gas, or your CO alarm sounds
- Your furnace is 15 or more years old and throwing codes regularly (see Furnace Repair or Replacement? How to Make the Right Call)
Our technicians service Lennox and all major furnace brands, carry common parts on the truck, and are available 24/7 for heating emergencies. We provide furnace repair in Highlands Ranch, furnace repair in Lone Tree, and throughout Castle Rock, Parker, Centennial and Englewood.
