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Fuel carrying something it should not — water, the wrong fuel, dirt, or microbial growth. The most damaging version is misfuelling: petrol in a diesel system removes the lubrication the high pressure pump depends on and can destroy it within a single journey.

The symptoms are erratic and affect everything at once, which distinguishes contamination from a component fault: several cylinders, several sensors, and pressure faults appearing together shortly after a fill. A fault that began at a specific refuelling is a strong lead, and worth asking about directly.

Diesel grows microbial contamination in the water layer at the bottom of a tank, producing a dark sludge that blocks filters repeatedly — a vehicle that eats fuel filters has this until proven otherwise. Draw a sample from the tank and look at it: water separates visibly, dirt settles, and diesel bug is unmistakable. Draining and cleaning the tank is part of the repair, since new filters into a contaminated tank block again.