How old is my Lennox?
Type your Lennox serial number below to get the month and year it was built. 1 documented Lennox serial format, covering 1973 to today.
The serial number is on the metal rating plate. On a furnace it is usually inside the front door on the side wall; on an outdoor AC or heat pump it is on the back or side of the cabinet, near the refrigerant lines.
Type the Lennox serial exactly as printed. Spaces, dashes and lowercase are all fine.
Lennox serial number formats
Plant, year, month letter
1973 to present
Digits 1-2 are the plant, not the date. This is the single thing most online Lennox decoders get wrong. Digits 3-4 are the year and the 5th character is a month letter.
5899L17212 reads as November 1999
Source: Building Intelligence Center; Dr. Ray Franco, PhD, PE (electrical-forensics.com) · checked 2026-08-25
Where to find the serial on a Lennox
- Gas furnace: open the front door. The rating plate is on the inside side wall, usually near the top.
- Outdoor AC or heat pump: a metal or foil plate on the back or side of the cabinet, near where the copper lines enter.
- Mini-split: on the side of the outdoor unit, and on the indoor head behind the front flap or on its right side.
- Photograph the plate rather than copying it by hand. Misreading 0 for O, 1 for I or 5 for S is the most common reason a lookup fails.
Is your Lennox showing an error code?
Age tells you where the unit sits in its life. The fault code tells you what is wrong right now. We have 44 Lennox codes, each quoted from the manufacturer's own manual.
HVAC Code Lookup publishes these decoding rules from public industry references, not from Lennox. If the rating plate prints an explicit manufacture date, trust the plate over any decoder.