How old is my A.O. Smith water heater?

By Dana Whitlock. Updated October 2026. Widely documented.

A.O. Smith moved to plain numbers around 2008, which makes newer tanks easy. The first two digits are the year and the next two are the production week. A serial beginning 1349 came off the line in week 49 of 2013, early December.

Older tanks are trickier. They used letter codes for the month, and the layout changed more than once. That is why this brand carries the Widely documented label. A.O. Smith has confirmed the year-and-week reading in recall notices, but it has never published a decoder for every era, so the older rules come from federal recall lists and inspector references.

Where to find the label on a A.O. Smith water heater

A.O. Smith places the data plate on the front of the tank, beside the gas control valve on gas models or near the thermostat cover on electric ones. Shine a flashlight across it at a low angle if the print has faded. Read it in place. There is no reason to unscrew the thermostat cover or touch the gas control, and electric covers should stay on while the breaker is on. A phone photo with the flash off often reads better than the naked eye.

How to read the code

Start by looking at the first character. If it is a digit and the first four characters are all digits, read year then week: 0805 is week 5 of 2008, 1503 is week 3 of 2015. If the serial starts with one letter followed by two digits, the letter is the month (A is January through M is December, with I skipped) and the digits are the year, so L07 means November 2007. A serial with two letters before the year digits, such as ME98, comes from the 1990s to 2004 layout: the first letter is a plant or line code, the second is the month. Very old tanks printed something like 800-M-72-09519, where the lone letter between dashes is the month and the next two digits are a 1900s year (December 1972 in that case).

Real serials in this code

When the format changed

The changeover dates are approximate because A.O. Smith never published them as a timeline. Federal recall lists show plant-plus-month codes like MA04 still in use in early 2004 alongside plain month-letter codes like G04, and gas tanks were still stamped L07A and M07A at the end of 2007. Electric tanks with year-week serials beginning 08 appear by mid-2008. In the overlap years, the presence of a leading letter is what tells you which rule to use. Watch for one trap on the oldest plates: a prefix such as M06 in M06-M-78-01849 is a code. It does not mean 2006. If the reading looks off, compare it with any date on the plate, the install receipt, or A.O. Smith's warranty verification page.

Who makes A.O. Smith

A.O. Smith was founded in Milwaukee in 1874 and is still headquartered there. Its residential water heater family includes State, Reliance, American, U.S. Craftmaster and John Wood, and the company also builds Whirlpool-branded tanks through American Water Heater. State and Reliance shared A.O. Smith's month-letter code before 2008 and its year-week code after. American, U.S. Craftmaster and Whirlpool used a year-week layout much earlier, back into the 1980s. A 2019 recall covered tanks sold under seven of these names, all identified by the same first-four-digit year and week.

How sure is this?

Widely documented. A.O. Smith doesn't publish this code; it comes from recall notices, service literature and inspector references. We tested the ao smith rules against 34 serials, 32 of them with a date known from a recall notice, a printed label date or the maker. None decoded to a wrong year; codes whose year mark repeats show every possible reading. Always confirm with A.O. Smith's serial lookup before you spend money on the answer.

Quick notes

• Digits first means year and week. A letter first means a month letter, then the year.
• Week numbers run 01 to 53. Divide by about 4.3 to estimate the month.
• Kenmore tanks built by this family use the same codes, so the same rules apply.
• Check the serial against A.O. Smith's warranty verification tool before paying for a major repair. It confirms both age and remaining coverage.