The final line of defence for the high pressure pump is the fine filter, which traps any debris before it can clog either pump or injectors.
The fine filter element – most often a throwaway item, like an oil filter – should be changed every 250-300 hours. The frequency of change depends very much on the cleanliness of the fuel used and the effectiveness of the prefilter. If in doubt change when the oil filter is changed. Take care to ensure that the seals on the element or bowl, and the holding bolts, are in good condition and placed correctly. Only fit the manufacturer’s recommended replacement filter, matched to the pump’s requirements.