Methodology
How rules are sourced, how they are tested, how confidence is scored, and the things this tool deliberately will not do.
The single rule everything else follows from
We do not publish a decoder rule we cannot cite. Not a plausible pattern, not a rule inferred from a handful of examples, not a format someone posted in a forum. If we cannot point at documentation, the rule does not ship and the brand is listed as unsupported.
This costs coverage. There are brands we could plausibly guess at and do not. We think a tool that returns nothing on a Samsung serial is more useful than one that returns a confident wrong year.
What every rule must carry
Each rule in the codebase is a typed object, and the test suite fails the build if any field is missing.
- Rule ID
- A stable identifier shown with every result, so a specific reading can be reported and traced.
- Manufacturer and brand family
- The brand, and the corporate family that actually built the appliance. These differ more often than people expect.
- Appliance categories and regions
- Which product types and markets the rule applies to. A rule scoped to North America is not applied to a European appliance.
- Production era
- Valid-from and valid-to dates. This is what stops a post-2006 Maytag rule claiming a 1995 appliance.
- Serial pattern and model pattern
- The exact shapes the rule accepts. A serial that does not match is reported as unrecognised rather than forced through.
- Decode logic
- A pure function returning every plausible reading, not just the first match.
- Confidence and caveats
- A baseline confidence, plus the known limitations shown to every user of that rule.
- Sources
- At least one reference, with an organisation, a URL and the date we last checked it.
- Test examples
- Positive, negative and — where the format repeats — ambiguous cases, all executed on every build.
- Review date
- When a maintainer last read the rule against its sources.
How we rank evidence
Sources are ranked. A rule supported only by the bottom tier does not ship.
- 1Official manufacturer documentation
- 2Manufacturer technical or warranty documentation
- 3Manufacturer-authorised parts or service documentation
- 4Established appliance repair organisation
- 5Credible technical reference
- 6Community reference (supporting evidence only)
Where two credible sources disagree, we keep both interpretations, return both candidates, explain the disagreement, and lower the confidence. We do not quietly pick the one we prefer.
How a lookup runs
- Normalise. Trim, uppercase, remove separators. Characters that are easily misread are flagged, never substituted.
- Select rules. Every rule for that manufacturer, filtered by appliance category, region, model pattern and production era.
- Match. All applicable rules are tried against the serial, not just the first that matches.
- Validate.Decoded values are checked for possibility. Week 99, month 13, week 0, a date in the future, or a year outside the rule's era all cause the candidate to be discarded.
- Expand ambiguity. Where a code repeats, every year it could mean is generated, most recent first.
- Score. Each candidate is scored on evidence for and against — model agreement, category agreement, region, purchase year, and how many years share the same code.
- Explain. Candidates are returned in confidence order with the supporting and conflicting evidence attached.
How confidence is scored
Internally each candidate gets a number, used only for ordering. What you see is one of four bands — high, moderate, low, or more information required — always with the reasons listed. We do not show percentages, because a figure like 94.72% implies a precision this data does not have.
Confidence goes up when:
- the serial matches a documented format exactly;
- the model number agrees with the expected pattern;
- the appliance category and region match the rule's scope;
- a supplied purchase year is consistent with the decoded date.
It goes down when:
- the code repeats and several years fit equally well;
- no model number was supplied;
- the model number contradicts the format;
- a supplied purchase year predates the decoded manufacture date;
- more than one rule produces conflicting readings.
What we will not do
- Claim a day when the format encodes a month.GE encodes a month. We say “March 2014”, never “12 March 2014”.
- Pick one year when four fit. If a code repeats, you get the shortlist.
- Substitute an ambiguous character. If swapping an O for a 0 would change the answer, we ask rather than assume.
- Name a factory we cannot source. Several formats include a plant code with no public table behind it. We label the character and stop there.
- Infer condition, safety, authenticity or warranty from a date. A serial number carries none of that.
Current rules and review dates
- whirlpool-standard-1990Whirlpool two-letter prefix with production week· 2 sources · reviewed 29 July 2026
- maytag-whirlpool-system-2006Whirlpool two-letter prefix with production week· 2 sources · reviewed 29 July 2026
- maytag-legacy-24-yearMaytag 24-letter year and month code· 2 sources · reviewed 29 July 2026
- kitchenaid-whirlpool-systemWhirlpool two-letter prefix with production week· 2 sources · reviewed 29 July 2026
- amana-whirlpool-system-2006Whirlpool two-letter prefix with production week· 2 sources · reviewed 29 July 2026
- ge-two-letter-month-yearGE two-letter month and year code· 3 sources · reviewed 29 July 2026
- hotpoint-us-ge-systemGE two-letter month and year code· 2 sources · reviewed 29 July 2026
- frigidaire-electrolux-systemElectrolux ten-character serial with year digit and week· 2 sources · reviewed 29 July 2026
- electrolux-na-systemElectrolux ten-character serial with year digit and week· 2 sources · reviewed 29 July 2026
- kenmore-whirlpool-builtWhirlpool two-letter prefix with production week· 4 sources · reviewed 29 July 2026
- kenmore-ge-builtGE two-letter month and year code· 5 sources · reviewed 29 July 2026
- kenmore-frigidaire-builtElectrolux ten-character serial with year digit and week· 4 sources · reviewed 29 July 2026
- bosch-fd-numberBosch FD number· 2 sources · reviewed 29 July 2026