Lennox Serial Number Lookup
Lennox serials hide the date in the middle. The 3rd and 4th digits are the year and the letter after them is the month, so 5806K… means October 2006. Enter yours to decode it.
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Serial number age lookup
Where to find the serial number on Lennox equipment
- Outdoor AC or heat pump: on the side of the cabinet, normally on or near the panel covering the electrical connections. Lennox labels are usually silver or white with black print.
- Furnace: inside the blower compartment door, on the side wall of the cabinet. Turn the power off at the furnace switch before opening it.
- Air handler and indoor coil: on the cabinet side or inside the access panel. The indoor coil often has its own plate and its own serial.
A modern Lennox serial is ten characters: four digits, a letter, then five digits, like 5806K12345. Lennox model numbers start with letters and include the capacity, such as a model beginning XC, EL or ML, so they’re easy to tell apart.
How the Lennox date code works
- Digits 1–2: plant code. These identify the factory and aren’t part of the date. Read past them.
- Digits 3–4: year.
06is 2006. - Character 5: month letter. A for January through M for December, without I.
- Last five digits: production sequence.
- Year
- Month
- Not part of the date
Worked examples
| Serial | Built |
|---|---|
| 5806K12345 | October 2006 |
| 1919C54321 | March 2019 |
| 5814H67890 | August 2014 |
| 6002A11111 | January 2002 |
| 1912M22222 | December 2012 |
Example serials are made up to show the format; they don't belong to real units.
| Letter | Month | Letter | Month |
|---|---|---|---|
| A | January | G | July |
| B | February | H | August |
| C | March | J | September |
| D | April | K | October |
| E | May | L | November |
| F | June | M | December |
The most common mistake with Lennox serials is reading the first two digits as the year. A serial like 5806K… isn’t from 1958; the 58 is the plant code. Always use the two digits right before the letter.
Older Lennox serials
Lennox equipment from the 1960s to the early 1990s used other layouts, including ones that start with a letter or that encode the day of the month. The decoder will say if it doesn’t recognize yours. A unit built in that era is far past its expected life, so the action is the same either way: plan a replacement.
How long does Lennox equipment last?
| Equipment | Typical life |
|---|---|
| Central air conditioner | 12–17 years |
| Heat pump | 10–15 years |
| Gas furnace | 15–20 years |
Lennox, founded in Iowa in 1895, sells mainly through its own dealer network, and its top lines (the Dave Lennox Signature Collection and Elite series) use variable-capacity compressors and communicating controls. That technology is efficient and comfortable, but it means repairs can need Lennox-specific parts and a technician who knows the system. Keep that in mind on an older high-end unit: the repair bill can be higher than on a basic single-stage system.
Signs a Lennox system is nearing the end
- The outdoor unit runs constantly in hot weather but the house doesn’t cool
- Frequent refrigerant top-ups (a leak somewhere in the system)
- Communicating-control error codes that keep coming back after repairs
- On a furnace: rust or cracks around the heat exchanger, flame rollout, or a burning smell from the cabinet
What to do with your result
Under 10 years: Lennox’s standard parts warranty usually depends on the unit being registered. Check your paperwork before paying for a part. Keep up annual maintenance.
10 to 15 years: expect more frequent repairs. For a major one, such as a compressor, an outdoor coil or a variable-speed blower motor, price a new system before saying yes. Lennox high-efficiency parts can be costly.
Over 15 years: plan a replacement. If you’re replacing only the outdoor unit, make sure the indoor coil is a matched replacement as well. Mismatched equipment can lose efficiency and may not be covered by the new warranty.
Frequently asked questions
How do I read a Lennox serial number?
Skip the first two digits, which are the plant code. The next two digits are the year and the letter after them is the month, from A for January to M for December, skipping I. 5806K12345 was made in October 2006.
How old is my Lennox furnace?
Find the serial on the rating plate inside the furnace's blower door (turn the power off first) and type it into the tool. The two digits before the letter are the year.
Why do Lennox month letters skip I?
I is easily mistaken for the number 1 on a stamped plate, so it's left out. That makes J September, K October, L November and M December.
How long does a Lennox AC last?
About 12 to 17 years for a central air conditioner and 15 to 20 years for a gas furnace. Lennox's variable-capacity models can run quieter and more efficiently but have more electronics to service.
Do Armstrong, Ducane and AirEase use the Lennox serial format?
Those brands belong to Allied Air, a Lennox company, but their serials can differ. We'll cover them on a separate page once those formats are verified.
Sources and accuracy
The Lennox decoder is built from these references and cross-checked between them. Facts are restated in our own words; any mistakes are ours.
Status: being verified against real data plates. Rules last reviewed 2026-10-18; page updated 2026-10-20. Not affiliated with Lennox. Report a wrong result.
Part of the hvac age lookup
This page covers one brand. For all furnace, air conditioner and heat pump brands, go to the hvac age lookup.