A part number looks like a meaningless string. It almost never is. Most major manufacturers encode the assembly group, the variant and the supersession history into the number itself. Once you can read the format, the catalogue starts telling you what the part is before you’ve looked it up.
OEM numbers: the canonical form
An OEM number is the manufacturer’s own identifier for a part. It is the source of truth for every other identifier downstream. Most aftermarket and brand-equivalent numbers ultimately cross-reference back to one.
VAG (VW, Audi, SEAT, Škoda)
Eleven characters, usually printed in three groups: three characters, three digits, three characters, optional one or two-character suffix. The first three characters identify the assembly group and originating brand. The middle block is the main number. The trailing letter is the revision — A, B, C and so on, each one a successor to the last.
03L 906 051 A
^^^ ^
| +-- Revision letter (A = first revision)
|
+-- Engine code prefix (03L = 2.0 TDI CR family)BMW
Eleven digits in two groups, no letters in the main number. The first two digits are the main group. The next three are the sub-group. The remaining six are the consecutive number. A trailing extension after a dot indicates regional or specification variation.
Ford
Three formats live in the parc simultaneously. Finis codes (six or seven digit, used for European service parts), engineering numbers (the old prefix-base-suffix format like 1S7G-9F593-AA) and World Service Numbers. A repairer searching just the finis code while the supplier indexes only the engineering number is a textbook reason why a graph beats a list.
PSA (Peugeot, Citroën, DS, Opel/Vauxhall post-2017)
Mostly four-character prefix plus three-digit suffix in the older numbering, transitioning to a longer Stellantis-era format on newer vehicles. The older format encodes the system family in the first character.
Vauxhall pre-2017 (GM Europe)
Seven or eight digit numbers, no letters. Filed under the Vauxhall part number in the UK and under the equivalent Opel number on the continent. Same physical part, two parallel number ranges. A buyer in Dover and a supplier in Calais can be holding the identical part and not know it.
Supersessions: history embedded in the number
When a manufacturer revises a part, the new revision usually inherits the base number with a new suffix. The old revision continues to exist in the parc, on every vehicle that left the factory with it, but stock against the old number gradually dries up as the new revision is supplied in its place.
A correct cross-reference treats the whole supersession chain as one group: 03L 906 051 → 03L 906 051 A → 03L 906 051 B. A search on any of the three returns stock against any of them, ranked by current availability.
Brand-equivalent numbers
A "brand-equivalent" number is the same physical part sold under a different manufacturer’s identifier — most often the original component supplier’s own number. Bosch, Continental, Valeo, Mahle, ZF, Pierburg, Hella and Denso all manufacture parts to OEM specification and sell them under their own brand box. The OEM number on the VW shelf and the Bosch number in the box on the same shelf at a factor are the same fuel injector.
Brand-equivalent numbers tend to be longer, broken into three groups of digits, and often include spaces. They almost never share a format with the OEM number, which is why catalogues miss the link without an explicit cross-reference.
Aftermarket numbers
An aftermarket-only brand — Febi, Blue Print, Borg & Beck, First Line — uses its own catalogue numbering. There is no relationship to the OEM number, no encoded meaning. The number is simply an index into that brand’s catalogue. Cross-referencing is the only route from an OEM number to an aftermarket equivalent.
Regional and date-coded variants
Two cars built in the same week to nominally the same spec can carry different part numbers for the same component because of regional homologation, supplier rotation on the line, or a running change. The number on the door card sticker of a UK-built car is not always the number a Belgian factor will index. Trace the part by its function and its specification, not by trusting a single number to be globally unique.
How to read an unfamiliar number
- Identify the manufacturer or brand the number originates from (prefix, length, separators).
- Strip whitespace and dashes for matching; preserve them for display.
- Note any trailing revision letter or suffix — it tells you where the part sits in the supersession chain.
- Resolve to the canonical group rather than relying on any single identifier.
- When in doubt, check the evidence on the cross-reference, not the number itself.