A faulty control unit can often be repaired, and the original unit usually keeps its coding and its link to the vehicle and the immobiliser. A new or used unit, by contrast, normally has to be coded or matched by a garage. Before deciding anything, make sure the control unit really is the cause.
This comparison comes from ElektroTech GmbH, a master-certified vehicle workshop (Kfz-Meisterbetrieb) in Viersen, Germany, that tests and repairs electronic vehicle components. We will also tell you when replacing is the better choice.
Is the control unit really faulty?
A fault code that names a control unit does not prove that this unit is broken. The cause is often the power supply, a corroded ground connection, a fuse, a sensor or the wiring. The German motoring club ADAC quotes repair specialists who find that between a fifth and a third of the units sent to them are not faulty at all.
So have the supply voltage and ground measured directly at the connector first, ideally under load. Replacing a unit on suspicion costs money and fixes nothing if the fault is in the wiring. A first assessment over the phone, based on the fault code and part number, helps you decide, but it does not replace testing the unit.
Repair, new part or used part: the comparison
The three options differ mainly in how much work is needed after fitting.
| Repairing the original unit | New part from the manufacturer | Used part | |
|---|---|---|---|
| Coding and matching | Usually not needed, the unit keeps its software and vehicle data | Has to be coded to the vehicle, often with manufacturer software | Has to be adapted; software version and equipment must match |
| Immobiliser | Usually stays matched | Has to be matched again | Has to be matched again, not possible on every vehicle |
| Availability | You already have the part | Sometimes no longer available for older vehicles | Depends on the market |
| Risk | Prior damage such as water, fire or heavy corrosion can rule out a repair | Lowest technical risk | Unknown history and condition |
| Cost | Quote after diagnosis | Part price plus coding at a garage | Purchase price plus adaptation at a garage |
According to the ADAC, refitting your own repaired unit saves the cost of coding a new part, and in the best case the saving compared with a new part can reach 90 percent. What it costs in your case depends on the component and the fault.
What do coding, matching and programming mean?
The three terms get mixed up, but they describe different jobs:
Programming (flashing): the unit's complete software is rewritten.
Coding: the software stays, only its configuration is set to the vehicle's equipment and variant.
Matching (adaptation): the unit learns a component or a key and takes over its values.
A new or used unit usually needs at least coding, and immobilisers also need matching. A repaired original unit brings its configuration with it.
When is repairing the original unit worth it?
Typical faults such as aged capacitors, cracked solder joints or failed power components can be repaired. Repair pays off especially when the new part is expensive or no longer available, when a swap would mean involved coding, for example for the immobiliser or gearbox, and when the vehicle is otherwise in good condition. Every repaired unit is also a new part that does not have to be made.
Testing shows whether your unit can be repaired. With the part number and the fault symptoms we can often tell you beforehand whether sending it in makes sense: +49 2162 8165472, Monday to Friday, 9:00 to 16:00 German time, in German or English, or send a repair enquiry.
When is replacing the better choice?
Replacing makes sense when the circuit board is badly damaged by water, fire or widespread corrosion, when testing shows the unit cannot be repaired economically, or when a suitable new part is cheap and your garage is coding it anyway. In those cases we tell you so plainly after the diagnosis.
What does a repair cost with us?
We do not quote a flat fixed price before testing, because every component and every fault is different. After the diagnosis you get an individual quote, and you decide whether we go ahead. The quote itself only costs something if that was expressly agreed.
If the unit cannot be repaired or you decline the repair, we send it back. For that we charge a small inspection fee plus return shipping. Standard repairs are usually done within 1–2 working days.
Before the unit goes back in: check these causes
A repaired or new control unit soon fails again if the cause stays in the vehicle. Before fitting, have these checked and fixed if needed:
short circuits in wiring and connected components,
a faulty power supply, for example a weak battery, corroded fuse holders or overvoltage from an incorrect jump start,
faulty or corroded ground connections,
moisture in the connector housing or where the unit is mounted.
A repair only lasts once the supply is clean.
Who does the coding after fitting?
Removal, fitting, coding and matching are only part of our job if expressly agreed. A qualified garage does this work following the vehicle manufacturer's specifications and our notes on the component. A repaired original unit usually needs no new coding. Whether basic settings are needed depends on the component.
Before sending a unit in, back up important data where you can and note any special settings. How to pack it and what goes in the box is in our guide sending in a control unit. The unit types we repair are listed under services.