The injectors fail above 100 000 kilometres, and how quickly depends on the quality of the fuel and on the servicing. Three paths lead here, and each one is checked differently at a viewing.

The nozzle can clog from poor fuel. As the high pressure pump wears, metal debris gets into the system and destroys the injectors; that can be seen in the fuel filter. And the Delphi injectors from before 2010 leak through a back-leak and dilute the oil with diesel; there is a back-leak test for that.

The symptom is an uneven idle, hard starting, jerking, lack of power and black smoke, along with a tapping noise and the injection warning light. In the fault memory the codes from P0201 to P0204 appear, cylinder by cylinder.

An injector that atomises poorly also fills the particulate filter faster, because it produces more soot. If an injector fails badly, the piston can even melt or burn through.

An injector costs around 100 pounds each, and it is worth replacing them together as a set. Reconditioned parts are also available, and they are noticeably cheaper.