Auto Parts Glossary: Key Terms Explained

A-Z of parts and catalog terms

EPC

Electronic Parts Catalog: the manufacturer's own digital parts database. It maps a specific vehicle to the components fitted at the factory, with exploded diagrams, genuine part numbers, quantities and the supersession history behind each number. It is the source a franchised dealer orders from.

VIN

Vehicle Identification Number: seventeen characters that identify one individual vehicle. It encodes the manufacturer, the build market and plant, the model year and a serial number, and it is the key every modern parts catalog is designed around.

WMI

World Manufacturer Identifier: the first three VIN characters, identifying the manufacturer and the country of build. A WMI that starts with W is a German build, J is Japan, 1, 4 and 5 are the United States.

VDS

Vehicle Descriptor Section: VIN characters four to nine, describing the model, body style, engine and restraint system. This is the block that separates two cars wearing the same badge.

VIS

Vehicle Identifier Section: VIN characters ten to seventeen, carrying the model year, the assembly plant and the serial number. The serial is what makes the VIN unique to one car.

Check digit

VIN character nine, calculated from the other sixteen using a fixed weighting. It is mandatory on vehicles built for North America and China and unused in many other markets, so a non-matching check digit on a European car is normal rather than a sign of a fake.

Frame number

An alternative vehicle identifier used in Japan and on Japanese-domestic-market imports, typically a model code, a hyphen and a serial — for example ZRE142-1234567. It is not a VIN and does not follow the seventeen-character structure.

Chassis number

In most markets, the last section of the VIN or the VIN itself; on older and Japanese-market vehicles, a separate identifier entirely. Which one a catalog wants depends on the market the car was built for.

Model year

The year a vehicle is designated as, encoded in VIN position ten. It is not the calendar year of manufacture: a car built in September 2025 is commonly a 2026 model year, which is why build date and model year can point at different part numbers.

Build date

The actual production date of a specific vehicle. Where a manufacturer changed a component mid-year, the catalog splits the part number by build date rather than by model year, and this is the field that decides.

Production split

A mid-year change in a fitted component. The catalog shows both numbers with a date or VIN range against each; the vehicle's own build date decides which applies.

Fitment

Whether a part matches the exact build in front of you, not merely the model. Fitment is decided by engine and gearbox code, market, equipment level and build date together.

PR code

A Volkswagen Group option code, printed on the vehicle data sticker and repeated in the service booklet. PR codes decide brake package, suspension variant, gearbox and dozens of equipment items, and they are what separates two otherwise identical entries in the catalog.

Data card

A manufacturer's record of how one vehicle left the factory, listing its option and equipment codes. Mercedes calls these SA codes; BMW, VAG and others keep equivalents. It is the authoritative answer when the catalog offers two plausible variants.

Option code

A short code identifying one factory-fitted option. A car's full option list is what turns a model into a specific build, and most catalogs use it alongside the VIN.

OEM

Original Equipment Manufacturer: strictly, the company that made the original part for the carmaker. In everyday use at a counter it usually means 'the correct factory part', which is a different claim — and that gap is where most miscommunication about parts begins.

OE

Original Equipment: the part as fitted on the production line. An OE part and a genuine part can be physically identical and differ only in the box they arrive in.

Genuine part

A part sold in the carmaker's own branded packaging with the carmaker's part number. It carries the manufacturer's warranty and is what a franchised dealer supplies.

OE-quality

An aftermarket part claimed to be built to a specification equivalent to the original. The claim is unregulated: it may describe the same factory's own-brand output, or it may describe a copy.

Aftermarket

Any replacement part not supplied through the original manufacturer's channel. Quality spans everything from the OE supplier's own-brand line to a poor copy, which is why the term alone tells you very little.

Matching-quality part

A term used in insurance and independent-repair contexts for a non-genuine part judged equivalent to the original. It is a commercial classification, not a technical one.

Reproduction part

A component remade to an original design, common for classic vehicles where the manufacturer no longer supplies the part. Tolerances and materials vary by maker.

Counterfeit part

A copy sold in packaging designed to pass as genuine. The catalog is the defence: a genuine number, checked against the manufacturer's own data, gives you something concrete to verify a supplier's claim against.

Part number

The manufacturer's identifier for one component. Its structure is house-specific: a VAG number is three groups of three plus an index, a Ford number is prefix-basic-suffix, a Toyota number is five digits and five.

Basic number

The middle block of a Ford part number, identifying what the component is regardless of vehicle: 1125 is a front wheel hub, 6731 an oil filter. It stays constant for decades while the prefix ages.

Index letter

The trailing letter or letters on a VAG or Ford part number, marking the revision level. A part revised from A to C changed twice, and on brake and suspension components the reason is often a failure in service.

Supersession

The replacement of one part number by a newer one. The old number is not wrong; it is out of date, and the catalog carries the chain forward to the number the factory supplies today.

Supersession chain

The full sequence of replacements behind a part number, sometimes several links long. Ordering a middle number may still find old stock — which is the superseded design, replaced for a reason.

Interchange

A cross-reference between a manufacturer's number and an aftermarket equivalent. Useful for sourcing, dangerous for identification: interchange tables are built by third parties and go stale.

Cross-reference

A mapping from one number to another, whether between manufacturers, between OE and aftermarket, or between old and current numbers. Always identify in the catalog first, then cross-reference to source.

NLA

No longer available: the manufacturer has discontinued the part and published no replacement. It is not the same as a superseded number, which does have a current equivalent.

Backorder

An order held until the manufacturer resupplies. A backordered part still exists in the catalog; an NLA part does not.

Exploded diagram

An assembly drawn apart so every component is visible in its fitted relationship, with an index number against each. It is how a catalog answers 'what else do I need' without you having to ask.

Assembly group

A section of the catalog covering one area of the vehicle — front brakes, timing cover, heating and ventilation. Manufacturers keep group numbering stable, which is why an experienced advisor can navigate an unfamiliar catalog quickly.

Callout number

The index number printed on an exploded diagram, mapping a drawn component to its row in the parts list. The number you order is the one in the list, not the callout itself.

Quantity per

How many of a component the assembly uses. The column most often skipped, and the reason a job stops for four bolts.

Applicability note

A remark limiting a part to certain builds, markets or date ranges. When two entries look identical, the applicability note is usually what separates them.

Main group

The top level of a catalog's structure, splitting the vehicle into major areas before you drill into assemblies.

Superseded-to

The forward pointer from an obsolete number to its current replacement. Following it to the end of the chain is the whole discipline of ordering from a live catalog.

Core charge

A refundable deposit on an exchange part, returned when the old unit goes back. It is a deposit, not a price, and quoting it as part of the price is a common source of customer disputes.

Exchange unit

A remanufactured component supplied against the return of the failed one, common on alternators, starters, turbochargers, steering racks and injectors. Frequently marked with an X suffix in VAG numbering.

Remanufactured

A used component rebuilt to a defined specification with wear parts replaced, sold under the manufacturer's own number and warranty. Not the same as a used or refurbished part.

Kit

Several components supplied under one number, often replacing parts once sold loose. Kits are where money quietly disappears: ordering a kit when only one item is needed, or ordering one item when the manufacturer now supplies only the kit.

Single-use fastener

A bolt, nut or seal designed to be fitted once and replaced — a torque-to-yield bolt stretches permanently on tightening. The catalog marks them; reusing one is a comeback waiting to happen.

Consumable

A gasket, seal, clip or fluid consumed by the repair. Catalogs list them against the assembly, which is why building the parts list from the diagram beats building it from memory.

Superseded stock

Inventory of a number that has since been replaced. It still sells, still fits physically, and is the design the manufacturer moved away from.

GRU

A suffix on Volkswagen Group body panels meaning the part is supplied primed for painting rather than finished.

X suffix

In VAG numbering, a marker that the part is a factory exchange unit, carrying a core deposit.

Coding

Writing configuration data to a replacement module so the vehicle accepts it. Common on batteries, injectors, steering angle sensors and control units — the part is correct but inert until coded.

Adaptation

Teaching a replaced component its operating parameters, such as a throttle body learning its closed position. Distinct from coding, and often required alongside it.

Calibration code

A value printed on a component, typically a diesel injector, that must be entered into the ECU after fitting. The part is identified by the catalog; the code comes off the part itself.

Immobiliser marriage

The pairing of a control unit to one vehicle's security system. A married unit cannot simply be swapped into another car, which is what makes some second-hand modules worthless.

Torque-to-yield

A fastener tightened past its elastic limit by design, giving a precise clamping force once and once only. Always a single-use part.

Homologation

Certification that a vehicle or component meets a market's regulations. It is why the same model built for two markets can take different lighting, emissions and restraint parts.

LHD / RHD

Left- and right-hand drive. Steering, dashboard, pedal box, wiper linkage, lighting and sometimes suspension and exhaust routing differ between them, which is why the build market matters more than the model name.

Market variant

The version of a model built for one region's regulations and conditions. Emissions hardware, lighting, cold-climate packages and corrosion protection all move with it.

ETKA

Elektronischer Teilekatalog: the Volkswagen Group's official catalog, covering Volkswagen, Audi, Škoda, SEAT and Cupra.

BMW ETK

BMW Group's official electronic parts catalog, covering BMW, MINI and Rolls-Royce, with eleven-digit grouped part numbers.

Microcat

An EPC platform built by Infomedia and licensed to several manufacturers; Ford is the best-known example, so 'Microcat' and 'Ford parts lookup' are used interchangeably in most workshops.

Mobis EPC

The Hyundai Motor Group's official catalog, covering Hyundai, Kia and Genesis, with five-five part numbers shared across the group.

Mopar

Stellantis's parts operation and catalog for Jeep, Dodge, RAM and Chrysler.

Dialogys

Renault Group's official catalog, covering Renault and Dacia, with ten-digit grouped numbers.

VIDA

Volvo's official parts and service information system, covering the Volvo car range by VIN.

ePER

The electronic parts catalog covering Fiat, Alfa Romeo, Lancia and Abarth.

Dealer licence

The agreement under which a manufacturer distributes its catalog software to franchised dealers. It is why installed copies of dealer EPCs circulating outside that channel are neither legal nor current.