BMW N47 diesel: A rough description of the thermowindow is enough

Germany’s Federal Court of Justice (BGH) ruled that a used-car buyer does not need to describe the technical details of a so-called thermowindow to pursue a damages claim against BMW. A general description of how the emissions system shuts down outside certain temperatures is sufficient.

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You don’t need to know the engineering to sue

Germany’s Federal Court of Justice (BGH — Bundesgerichtshof, the highest civil court in Germany) ruled on 2 June 2026 that a car buyer suing BMW over illegal emissions manipulation does not need to describe the technical details of the defeat device (case no. VIa ZR 482/23). It is enough to state that the exhaust gas recirculation system — the part of the engine that reduces harmful nitrogen oxide emissions — only works properly within a certain temperature range and is reduced or switched off outside that range. The Brandenburg Court of Appeal (Oberlandesgericht Brandenburg, case no. 4 U 1/22) had dismissed the claim because the plaintiff had not been specific enough. The BGH said that was wrong, and sent the case back for a fresh hearing.

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What this is all about — the diesel scandal in brief

If you moved to Germany from the US or elsewhere, you may not be familiar with the „Dieselgate” saga. Starting around 2015, it emerged that several car manufacturers — most famously Volkswagen, but also BMW and others — had built software into diesel engines that detected when the car was being tested for emissions and temporarily cleaned up its exhaust. On the road, the system worked differently. One common technique is the so-called thermowindow (Thermofenster): the exhaust gas recirculation system only runs at full capacity within a narrow temperature range (in this case, 17 to 33 °C). Below or above that window, it throttles down — meaning the car emits far more nitrogen oxides than allowed. German law and EU regulations treat this as an illegal defeat device. Buyers who were affected can sue the manufacturer for damages under German tort law (§ 823 BGB) — roughly the equivalent of a product liability or fraud claim. The typical remedy is unwinding the purchase: you hand back the car and get your money back (minus a deduction for how much you drove it).

A used BMW X3, bought in 2015

The plaintiff had bought a used BMW X3 xDrive 20d in January 2015 — a diesel with a so-called N47 engine, certified to the Euro 5 emissions standard. He claimed the car contained an illegal defeat device and sued BMW for full repayment of the purchase price in exchange for returning the vehicle. The lower courts — first the Potsdam Regional Court (Landgericht), then the Brandenburg Court of Appeal — threw out the case. Their reasoning: the plaintiff had not described the thermowindow precisely enough. The BGH disagreed and overturned that decision. The case now goes back to Brandenburg for a new hearing.