Lennox furnace 3 flashes: the meaning depends on the board

Furnace control display or LED

3 flashes

“Burner failed to light, or lost flame during heat demand”

EL195*NE, EL196*E, EL180*NE, ML180*, ML193*E, ML196*E · 507758-05 p. 43; 507764-06 p. 40; 507647-05 p. 24; 507945-02 p. 26; 507863-02 p. 39; 507966-04 p. 40
Technician

What it means

Three flashes of the red LED mean different things on different Lennox boards, so the count alone does not name the fault. The tables give these meanings:

Table	Series and documents	3 flashes, as printed	Source
13-flash tables, boards 103217-02, 103217-03 and 107163-01	Flash-code editions of the EL180, EL195, EL196, ML180, ML193 and ML196	“Burner failed to light, or lost flame during heat demand”	507966-04, pp. 40–41; 507758-05, pp. 43–44; 1121-L4, pp. 8–10
13-flash tables with no board number	EL195UH (2011), ML180UHV (2024)	“Burner failed to light, or lost flame during heat demand”	506596-01, p. 49; 507945-02, p. 26
9-flash tables	ML193UH (2013), ML195UH (2019), ML180DF (2018)	“Pressure switch open with combustion air inducer energized.”	507121-01, p. 39; 507268-03, p. 39; 507326-01, p. 21
ML296UHV installation instructions	ML296*V	“Pressure switch open with inducer energized”	507869-04, p. 46
ML296DFV(X) service literature	ML296*V	“Low-fire pressure, rollout or limit switch open.”	1904-L1, p. 8

The three 13-flash boards give 3 flashes the same wording. The documents name the boards, but none of them says which board a given model or serial number has 507966-04, pp. 40–41; 1121-L4, pp. 8–10. One series can appear in two rows: within the ML180 series, the 2018 ML180DF instructions have the 9-flash table and the 2024 ML180UHV instructions the 13-flash one, and the ML193 changed between the 2013 ML193UH and the 2024 ML193UHE instructions 507326-01, p. 21; 507945-02, p. 26; 507121-01, p. 39; 507863-02, pp. 39–40. Match your furnace to the document by series and edition: finding the rows for your furnace.

How a code looks: the notes under the 13-flash tables say the LED flashes X times at ½ second on and ½ second off, stays off for 3 seconds, then repeats 507966-04, p. 41. The ML180UHV table gives 1/3 second on and 1/3 second off 507945-02, p. 26. Not every blinking LED is a code. The EL180UHNE and EL195UHNE owner’s manuals call a slow flash, 1 second on and 1 second off, the state when the furnace is not in operation, and a continuous rapid flash the state during a call for heat and normal heating; the red LED can be seen through the viewport on the lower access panel 507648-02, p. 3; 507759-01, p. 4. On the 107163-01 board the heat-call “fast heartbeat” is also 0.5 second on and 0.5 second off; in the notes, what sets a code apart is the 3-second pause after the count 507966-04, p. 41.

Check these first

Lennox’s owner’s manuals have no flash code table and give no steps for 3 flashes. What they give is a general list for a unit that will not operate, to work through before calling a Lennox service technician. In the SLP99DFVK manual the order is:

Check that the electrical disconnect switches are ON 508590-01, p. 5.
Check the room thermostat for proper setting 508590-01, p. 5.
Replace any blown fuses or reset circuit breakers 508590-01, p. 5.
Check that the gas valve is ON 508590-01, p. 5. Its switch is behind the outer access panel; the manual’s gas valve steps set the thermostat to the lowest setting and turn off all electrical power to the unit before that panel comes off, and the switch is moved by hand only, never with tools 508590-01, p. 4.
Check that the air filter is not plugged, which would limit air flow 508590-01, p. 5. The SLP99DFVK manual says its filters are installed external to the unit, should be inspected monthly, and that a filter must be in place when the unit is operating 508590-01, p. 4. Before any service or maintenance, it says to turn the electrical power to the unit OFF at the disconnect switch 508590-01, p. 2.
Check that gas is turned on at the meter 508590-01, p. 5.
Check that the manual main shut-off valve is open 508590-01, p. 5.

Then record the model and serial number from the nameplate and call 508590-01, p. 5. The list is not tied to any code. More on it, and on how other Lennox manuals word it: owner checks before a service call.

The model number leads to the series and to its documents. It does not say which 13-flash board the furnace has; the documents do not give that 507966-04, pp. 40–41.

The manual says the furnace lights its burners automatically and not to try to light them by hand 508590-01, p. 4. Its Maintenance section says the maintenance procedures should only be conducted by a licensed professional service technician 508590-01, p. 5.

When to call a licensed technician

The flash tables have two columns, the count and its meaning, and no column of corrective actions 507966-04, pp. 40–41; 507121-01, p. 39. There is no technician list to repeat here. Call when:

The furnace still will not operate after the checklist 508590-01, p. 5.
The 3 flashes keep coming back. The owner’s manuals give no step for that; these pages send it to the technician.
The gas valve switch will not move by hand. Do not try to repair it 508590-01, p. 4.
You notice any of the signs in the manual’s “Safety Precautions”, such as new or unfamiliar sounds, unusual odors, or unusual dirt or rust on the vent pipe; the manual says to shut down the unit and contact a Lennox dealer 508590-01, p. 6.
Any work inside the furnace 508590-01, p. 5.

Notes for different models

The K editions of several series print E-code tables instead of flash tables, for example the ML196UHEK, the EL195UHNEK and the ML296DFVK 508569-01, pp. 44–46; 508575-02, pp. 45–47; 508521-01, pp. 41–44. The documents do not translate flash counts into E-codes.
On the ML296, a flashing green LED means “High heat demand, high pressure switch not closed” 507869-04, p. 46; 1904-L1, p. 7. That green LED is separate from the red LED that flashes the codes 507869-04, p. 46.
On the 13-flash boards, 5 flashes is “Watchguard - burner failed to light, exceeded maximum number of retries or recycles.” and 9 flashes is “Pressure Switch failed to close or opened during heat demand” 507966-04, p. 40. In the 9-flash tables, 7 flashes is “Lockout, burners fail to light.” 507121-01, p. 39.
The 2011 G61MPV board flashes two numbers with a pause between them, and “3 + 1” there is “Limit switch open.” 506506-01, p. 63.