Volkswagen DQ250 (02E)

The first dual-clutch gearbox sold in volume to ordinary drivers, and still the one most people mean by DSG. Six ratios, two clutches running in oil, and a torque rating around 350 Nm that put it behind almost everything transverse in the Volkswagen Group for more than a decade.

The clutches are wet, and that single fact separates it from the DQ200 it is constantly confused with. They run submerged in fluid that both cools them and carries away what they shed, which is why this unit tolerates traffic, towing and hill starts that punish a dry-clutch box. It also means the fluid is doing real work and is not optional maintenance: the interval is about 60,000 km, and a skipped one shows up as shudder when pulling away and hesitancy between the lower gears.

The mechatronic unit is where the money is. It is the valve body and the control electronics in one assembly, sitting inside the gearbox in the same fluid, and it fails in two distinct ways worth telling apart. Debris from tired clutches or an overdue fluid change jams the solenoids, which is a consequence of neglect and often recoverable. Electronic failure inside the unit is not, and it presents as gear selection that goes wrong suddenly rather than gradually — a gearbox that drops to neutral, refuses a gear or throws the car into limp mode without any preceding roughness.

Speed-sensor faults produce the same symptoms as something far worse, which is the practical reason to read the codes before agreeing to anything. The unit cannot shift what it cannot measure, so a failed sensor makes a mechanically sound gearbox behave like a broken one.

Judder that arrives only when the car is cold and clears as it warms is the clutch pack asking for fluid. Judder that stays is asking for clutches.