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How old is my Guardian furnace or AC?
Type the serial number from the Guardian label to see when it was built. Guardian units are built by York (Johnson Controls until 2025, now Bosch), so the serials follow that company's format.
Example
- NPlant: N = Norman, Oklahoma
- 0Year, first digit: 0
- DMonth: D = April
- 6Year, second digit: 6
Built April 2006
20 years, 6 months old
In replacement window. Inside the usual 15 to 25 year lifespan, so replacement could come any time. Worth budgeting for it.
How this was read
- The first letter, N, is the plant (Norman, Oklahoma).
- The second and fourth characters, 0 and 6, make the year: 2006.
- The third character, D, is the month: April. York skips I and J in month letters.
Format: York plant, year digits and month letter. Sources: CPSC 2001 York, Luxaire and Coleman furnace recall (serials WFKM and WGKM), McGarry and Madsen: York serial numbers, Building Intelligence Center: York, Inspector Handbook: York.
Reading a Guardian serial number
Guardian is a York brand. The serial uses York’s format: plant letter, a year digit, a month letter, then the second year digit.
About Guardian
- Guardian sells furnaces, air conditioners and heat pumps out of York’s Norman, Oklahoma operation.
- Guardian’s manufactured-home gas furnaces include the DGAX (80% AFUE) and MM9E (95% AFUE).
- Guardian was one of the brands in the residential HVAC business Bosch bought from Johnson Controls in 2025.
Sources: Guardian (ACHR News HVACR Directory), Guardian Manufactured Housing Furnaces and Air Conditioners, Johnson Controls International plc Form 8-K, Exhibit 99.1 (July 2024)
York plant, year digits and month letter
Used 2004 to today. Example: W1G7123456
The serial starts with a plant letter, a digit, a month letter and another digit. The two digits together are the year. W1G7 means July 2017: 1 and 7 make 17, and G is July. Month letters skip I and J.
Worked example: N0D6653823 was built April 2006, so it is 20 years, 6 months old as of October 2026.
| Letter | Month |
|---|---|
| A | January |
| B | February |
| C | March |
| D | April |
| E | May |
| F | June |
| G | July |
| H | August |
| K | September |
| L | October |
| M | November |
| N | December |
Sources: CPSC 2001 York, Luxaire and Coleman furnace recall (serials WFKM and WGKM), McGarry and Madsen: York serial numbers, Building Intelligence Center: York, Inspector Handbook: York
York four letters and six digits
Used 1971 to 2004. Example: WGKM007655
The serial starts with four letters. The first is the plant, the second is the month and the third is the year. The year letters repeat every 21 years, so A to N can mean two dates. WGKM means July 2001.
Worked example: WGKM007655 fits July 1980 or July 2001. The page above explains how to tell which.
| Letter | Month |
|---|---|
| A | January |
| B | February |
| C | March |
| D | April |
| E | May |
| F | June |
| G | July |
| H | August |
| K | September |
| L | October |
| M | November |
| N | December |
| Letter | Year |
|---|---|
| A | 1971 or 1992 |
| B | 1972 or 1993 |
| C | 1973 or 1994 |
| D | 1974 or 1995 |
| E | 1975 or 1996 |
| F | 1976 or 1997 |
| G | 1977 or 1998 |
| H | 1978 or 1999 |
| J | 1979 or 2000 |
| K | 1980 or 2001 |
| L | 1981 or 2002 |
| M | 1982 or 2003 |
| N | 1983 or 2004 |
| P | 1984 |
| R | 1985 |
| S | 1986 |
| T | 1987 |
| V | 1988 |
| W | 1989 |
| X | 1990 |
| Y | 1991 |
Sources: CPSC 2001 York, Luxaire and Coleman furnace recall (serials WFKM and WGKM), McGarry and Madsen: York serial numbers, Building Intelligence Center: York, Inspector Handbook: York