Combustion gases escape past the shaft of the Bosch piezo injector, and soot builds up around the shaft. The symptoms are a hard, rough cold start and a rough idle. The engine has six injectors, so finding the faulty one is more work as well. The quantity correction values of the control unit have to be read per injector, and this is worth doing before anyone replaces all of them. A cleaning additive and better quality fuel help to prevent the sooting. The fault comes up above 120,000 km. The high-pressure system of the same engine (Bosch CP3) can also give starting trouble and a lack of power at high mileage, so the diagnosis separates the two.