On this generation the injectors wear internally, and deposits build up on the nozzle needle from residual fuel and from the heat. A worn injector lets more fuel back into the return line than it should. The symptoms are rough running when cold, hard starting, sooting and a quiet knocking noise. The most reliable test is measuring the return quantity per injector. If the return of one injector stands out from the others, that injector is internally leaking. On the OM646 with the Bosch system the limit for the quantity correction is plus or minus 5 cubic millimetres per stroke: if an injector reaches that, it has to be replaced. This measurement says which unit is at fault faster than any fault code reading. The fault comes up above 180,000 km.