Many OM646 owners do not know that this engine has a timing belt, not a timing chain. The belt drives not only the camshaft but also the injection pump, and the water pump runs off it as well. It is an interference engine: if the belt snaps, the valves and the pistons meet, and the end of it is a four-figure repair or an economic write-off. The factory replacement interval is 200,000 km or 10 years, whichever comes first. Experienced workshops shorten this to 150,000 km, especially on a car with an unknown service history or one that does a lot of city driving. At the change it is worth replacing the tensioner and the idler pulley as well, because their condition cannot be seen by looking, and the water pump too, since the same belt drives it. The complete change is 500 to 900 euro at an independent workshop and 800 to 1,400 euro at a franchised one. This is the most important preventive job on this engine.