A used original part with the right OEM number is a great way to save money. But with modern cars, fitting the part is not always the last step. Many electronic parts store information about the car they came from, and they only work properly in your car after they have been coded, adapted or programmed.
This guide explains which parts usually need it, which don't, and how to plan for it before you order.
What coding means
Coding is done with a diagnostic tool connected to your car. Depending on the part, it can mean telling the part which equipment your car has, pairing it with the immobiliser, or entering values such as a correction code. Some of it can be done with common diagnostic tools, and some needs a dealer or a specialist workshop.
Requirements differ by make, model and production year, so treat the lists below as a guide, not a rule for every car.
Parts that often need coding or programming
- Engine control units (ECU) Usually linked to the immobiliser and the car's software. A used ECU normally needs to be programmed or matched before the engine will start.
- Keys, immobiliser and keyless modules Need to be paired with the car.
- Instrument clusters Often linked to the immobiliser and store the mileage, so they usually need to be adapted by a specialist.
- Airbag control units May store crash data and car-specific settings. They usually need coding, and some need a reset by a specialist first.
- Body, comfort and gateway modules Control lights, locks, windows and the communication between modules. They usually need to know your car's equipment.
- Radios, navigation and display units Some ask for a radio code, and many need coding to work with the rest of the car.
- ABS and ESP units Some need coding or a calibration after fitting.
- Gearbox control units Often need programming and an adaptation drive.
Parts that may need an adaptation
- Diesel injectors Many have a correction code printed on them that has to be entered into the engine control unit.
- Throttle bodies and some sensors May need a basic setting or adaptation so the engine runs smoothly.
- Headlights with control modules Adaptive or LED headlights may need coding or a calibration.
- Parking sensors and cameras Usually plug and play, but some systems need coding or a calibration.
Parts that usually don't need coding
Mechanical and simple electrical parts usually work right after fitting: body panels, trim, mirrors without electronics, pumps, hoses, coolers, brackets, housings, switches and most simple sensors.
How to plan for it
- Match the OEM number first. Coding can't make a wrong part fit. Check the number on your old part, and compare the side, connectors and equipment. Our guide on checking compatibility walks you through it.
- Ask your workshop before you order. Check whether they can code the part for your car, or whether it needs a dealer or a specialist, and what it will cost.
- Keep your old part. Its label and data can help your workshop with the coding.
- Ask us if you are unsure. Send us the OEM number or a photo of the label on your old part in the chat, and we will tell you what we know about that part.
Not sure where to find the number? Read how to find the OEM number of a replacement part.