What do Lennox furnace error codes and blinking lights mean?
A Lennox furnace reports faults as DS1 and DS2 LED flash patterns on its control board and as numbered alerts on S30 thermostats, such as 223 for a low pressure switch open and 250 for a primary limit open. Your furnace's own door chart is the authority for each pattern.
Key points
- Older SureLight boards use two LEDs, DS1 and DS2, and the combination of slow, fast, on and off is the code. Newer communicating furnaces also send numbered alerts to the thermostat.
- Lennox uses several boards. The chart on your furnace door or in its own manual is the authority, not any list on a website.
- Read the light through the sight glass with the door on, and count the pattern before anyone cuts power.
- Safe checks: thermostat, breaker and furnace switch, filter, snow on the PVC pipes, and the condensate line. One restart after that is reasonable.
- Limit, rollout and flame-sensed-with-no-gas faults are stop-and-call codes.
Read the control board's LED through the sight glass in the blower door, count the flashes of DS1 and DS2, and match them to the chart on your furnace door; on a communicating system, a number on the S30 thermostat such as 223 or 250 names the same fault. Do this with the door on, before anything is powered down. That count is what a technician will ask for first, and the tables below show how Lennox's codes are built, not a replacement for your own chart.
Where do these Lennox codes come from?
The codes below come from Lennox's own literature, and the documents are named so you can compare:
- Lennox SureLight Ignition System service literature, Corp. 0222-L7 (revised 11-2006). It covers the two-stage and two-stage variable-speed SureLight boards used in Lennox's G60-series furnaces and others of that era, with the DS1 and DS2 diagnostic chart.
- Lennox S30 Smart Thermostat Installation and Setup Guide, 507536-08 (3/2025). Its alert code table covers what a communicating thermostat reports for a gas furnace. The guide's own screens refer to the iComfort dealer app, which is why many homeowners call these iComfort codes.
Lennox has made many boards over the years. A newer high-efficiency furnace may carry a different control than the SureLight boards in that older document, so treat the tables below as examples of how Lennox codes work and check your own model's manual for the final answer.
How do you find the chart for your furnace?
Many Lennox furnaces have a diagnostic label on the back of the blower door. If yours is missing or unreadable, the model number on the rating plate gets you the installation or service manual from the Lennox website, with the code table in its troubleshooting section. Write down the model number, the pattern and what the furnace did before you call; it shortens any phone call, including ours.
How do you read the code safely?
Lennox doors often have a small sight glass, so you can watch the board's LED with the door on and the furnace powered. On a SureLight board, note which of DS1 and DS2 is flashing and whether each flash is slow or fast. Write the pattern down and photograph the door label before anyone cuts power. Lennox literature says the control stores the last ten codes for a technician to review, and describes clearing them by shorting terminals on the board, which is technician-only work.
What do the DS1 and DS2 lights mean?
The flash-code table comes from Table 5 of Lennox Corp. 0222-L7, for SureLight boards 18M3401, 46M9901, 18M9901 and 49M5901. Lennox defines a slow flash as one per second and a fast flash as three per second.
| DS1 | DS2 | Lennox's description |
|---|---|---|
| Slow flash together | Slow flash together | Power on, normal operation (also during cooling and continuous fan) |
| Fast flash together | Fast flash together | Normal operation, heating demand from the thermostat |
| Slow flash | On | Primary, secondary or rollout limit open. Limits must close within 3 minutes or the unit goes into a 1-hour Watchguard |
| Off | Slow flash | Low pressure switch open, blocked inlet or exhaust vent, or switch closed before the inducer started |
| Off | Fast flash | High pressure switch open, blocked inlet or exhaust vent, or switch closed before the inducer started |
| Alternating slow flash | Alternating slow flash | Watchguard: burners failed to ignite, limit open over 3 minutes, flame lost 5 times, or pressure switch opened 5 times in one cycle |
| Slow flash | Off | Flame sensed without the gas valve energized |
| Fast flash | Slow flash | Main power polarity reversed |
| Slow flash | Fast flash | Low flame signal (below 0.2 microamps); Lennox says to replace the flame sense rod |
| Alternating fast flash | Alternating fast flash | During ignitor warm-up: improper ground, broken or open ignitor, or line voltage below 75 volts |
Some boards in the same document, numbers 100869 and 100870, use a single red LED that flashes in two-part codes. It flashes a number of times, stays off for two seconds, then flashes a second number of times. If your furnace has that board, the chart on the door matters even more.
What do the iComfort and S30 alert numbers mean?
On a communicating Lennox system the furnace control sends numbered alerts to the thermostat. These are the gas furnace entries from the S30 guide's alert table:
| Alert | Lennox's text | Plain meaning |
|---|---|---|
| 200 | GF Rollout Limit Switch Open | Flame rolled out of the burner area, or the vent is blocked. Service urgent |
| 223 | GF Low Pressure Switch Open | Inducer pressure not proven at startup |
| 224 | GF Low Pressure Switch Stuck Closed | Switch closed when it should be open. Lennox notes older communicating thermostats can post this briefly if a new call for heat starts within 30 seconds of the last one |
| 225 | GF High Pressure Switch Failed to Close | High-stage pressure not proven |
| 226 | GF High Pressure Switch Stuck Closed | High-stage switch will not open |
| 227 | GF Low Pressure Switch Open in Run Mode | Pressure lost while running |
| 240 | GF Low Flame Current, Run Mode | Weak or lost flame signal |
| 241 | GF Flame Out of Sequence, Still Present | Flame sensed with no call for gas heat |
| 250 | GF Primary Limit Switch Open | Overheating, usually airflow. Soft lockout after 3 minutes |
| 270 | GF Flame Failed to Ignite | Watchguard after five failed tries in one call |
| 271 | GF Low Pressure Switch Open (Watchguard) | Five pressure-switch strikes in one call |
| 273 | GF Flame Fail in Run Mode | Flame dropped out during the cycle, Watchguard |
Filter reminders are separate. The guide lists service notification 3000 as Filter 1 and 3001 as Filter 2, which is a maintenance reminder and not a fault. Our post on communicating HVAC systems explains why these thermostats report so much detail.
What do the common code families usually mean?
Pressure switch. The control never saw proof that the inducer is moving exhaust. Around here the usual causes are frost or snow on the PVC pipes, a frozen or plugged condensate drain, or a cracked pressure hose, and Lennox's own tips for these alerts name the same three. Our furnace pressure switch guide goes through them in order.
Flame sense. The burners lit, then the control could not confirm flame. Lennox's tip for alert 240 is to clean or replace the flame sensor and confirm a good ground. See our flame sensor post.
Limit. The furnace overheated and shut the burners off. Lennox's tips for alert 250 point to low supply air, such as a dirty filter or blocked ductwork. Our limit switch post covers it.
Ignition lockout. The Watchguard codes mean the furnace tried repeatedly and stopped. The chart says why.
Rollout. Flame came out where it should not. Lennox says the cause of the trip must be corrected. That is a technician job.
What can you safely check yourself?
Lennox's tips for the pressure-switch and limit alerts point to checks that need no tools and no panels removed. The basics are in our general furnace light codes guide: thermostat, breaker and furnace switch, snow and frost on the PVC pipes, and water at the furnace. Replace a gray filter, since a clogged one is the first suspect for limit alert 250. Then restart once and watch the next cycle. If the same code returns, the condition is still there.
When should you call?
Book furnace repair if the same code returns after one restart, if the code is a limit, rollout or flame-sensed-without-gas fault, if the light is dark with power on, or if you cannot find a chart for your board. Read us the model number, the pattern and any alert number. Our Lennox service page covers what we work on, and no-heat calls get priority, including furnace repair in Minneapolis. You get a written flat-rate price before work begins. Call 612-429-3465. For other brands, see our furnace code overview.
Common questions about heating
Why does my Lennox thermostat show alert 250?
Alert 250 is Lennox's Primary Limit Switch Open in the S30 alert table, usually overheating from poor airflow. Check for a dirty filter or closed registers first; the soft lockout clears after the furnace cools, and a code that returns needs a technician.
What does it mean if both lights on my Lennox board flash together?
On the SureLight two-stage boards covered in Lennox service literature Corp. 0222-L7, both lights flashing slowly together is normal power-on operation, and both flashing fast together is normal while the thermostat is calling for heat. Both flashing slowly in alternation is a Watchguard lockout. The pattern is only as meaningful as the chart for your exact board, so check your door label.
Does Lennox service notification 3000 mean my furnace is broken?
No. The S30 guide lists notifications 3000 and 3001 as filter reminders, which are maintenance prompts and not furnace faults. Replace the filter and reset the reminder on the thermostat. If a different alert number shows up alongside it, or the furnace is also short of heat, read that number against the S30 alert table and treat it separately.
My Lennox thermostat shows a service alert number. Is that the same as the blinking light?
It is related but not identical. On communicating Lennox systems the furnace control reports numbered alerts to the thermostat, such as 223 for a low pressure switch open or 250 for a primary limit open, as listed in the Lennox S30 thermostat guide. The board LED or seven-segment display on the furnace still shows its own code, and a technician reads both.