4 min
"We'll go to a repair shop, connect the diagnostics, and we'll immediately see the real mileage." This is probably one of the most frequently heard pieces of advice when buying a used car. And while there is some truth to it, the reality is much more complex.
Today, diagnostics alone are often no longer enough. Experienced fraudsters know exactly where the mileage is stored, which control units need to be adjusted, and how to avoid leaving obvious traces.
Nevertheless, BMW, Audi, and Mercedes-Benz cars share one common feature – they do not store mileage in just one place. That is exactly why professionally performed diagnostics can still help detect discrepancies, especially if the mileage was altered hastily or incompletely.
Why specifically German cars?
It is no secret that BMW, Audi, and Mercedes-Benz are among the most frequently clocked cars in Europe. The reason is simple – money. The exact same five- or six-year-old premium class car with 120,000 kilometers on the odometer can cost several or even a dozen thousand euros more than an identical model that has driven 300,000 kilometers. Therefore, the investment into mileage correction pays off very quickly for some dishonest sellers.
The speedometer alone has long been insufficient
Since around 2005–2010, many BMW, Audi, and Mercedes models began storing mileage in several electronic control units simultaneously. Depending on the model, mileage information can be recorded in the dashboard, the engine control unit, the automatic transmission controller, the ABS system, the electronic ignition system, or other modules. Theoretically, this was supposed to complicate the fraudsters' work. Practically – it complicated it, but did not stop it. Experienced specialists can adjust the information in many blocks at the exact same time. However, cheap or rushed mileage rollbacks often leave discrepancies that can be detected during a professional diagnostic scan.
BMW: More than just one number
BMW cars are famous for their complex electronic architecture. Depending on the model and the year of manufacture, mileage information can be stored in the dashboard, the engine control unit, CAS or FEM/BDC systems, the automatic transmission controller, and other modules. In some models, additional information may also be present in the electronic key; however, it is important to understand that this is not a universal or always reliable source. Not every key stores the latest data, so you should not rely on it alone. If different modules show different mileage, it is already a serious signal that the car's history is worth investigating deeper.
Audi and the entire VAG Group
Audi, just like other Volkswagen Group cars, also stores mileage in several electronic units. Professional diagnostics allow for a comparison of the information found in the dashboard, the engine control unit, the automatic transmission controller, and other systems. Sometimes it is also possible to analyze indirect operational indicators – for example, the parameters of the diesel particulate filter (DPF) or other wear data. They do not show the exact mileage, but they can help evaluate whether the car's general condition matches the declared kilometers. This is not definitive proof, but it is another piece of the puzzle.
Mercedes-Benz: Electronics remember a lot
In Mercedes-Benz models, an important role is played by the electronic ignition system (EZS/EIS), which in certain models also stores information about the vehicle's operation. In addition, professional STAR diagnostics allow for a comparison of data from various control units to detect discrepancies. In some models, additional clues can be provided by the factory service system, maintenance history, or other electronic modules. That is why Mercedes diagnostics are usually carried out using the manufacturer's own equipment – universal diagnostics do not always reach all the necessary data.
Can diagnostics show the absolute truth?
No. This is one of the biggest myths in the used car market. If the mileage correction was done professionally and all the most important control units were changed, the diagnostics might not show any discrepancies at all. However, a large portion of cars are not adjusted so meticulously. In such cases, a professional technician often notices discrepancies between different modules or other signs that raise suspicion of fraud.
Why are diagnostics alone still not enough?
Diagnostics show what the car's electronics currently "remember." However, they will not answer other important questions. Has the car been operated in several countries? Has it been involved in traffic accidents? What was its mileage five years ago? Were there any insurance claims? Was the car serviced regularly? These questions are no longer answered by the car's computer, but by its history. Therefore, the best result is achieved through a combination of several methods – professional diagnostics, document checks, a physical vehicle condition assessment, and a VIN history report. Each of these reveals a different part of the car's life, and together they allow you to form a much more accurate picture than by trusting just the number displayed on the dashboard.
