Cause
Low fuel level causing poor heat dissipation
A tank too low to absorb the heat returned to it. A returning diesel system recirculates far more fuel than it burns, and the tank is the heat sink — a large volume of cool fuel carries away what the pump and rail put in.
Run the tank low and the same heat goes into a much smaller volume, so fuel temperature climbs steadily on a long run and the module reports it. The signature is a fuel temperature fault appearing only near the bottom of the tank and clearing after a fill, which is why it is worth checking the level before the sensor or the cooler. A blocked fuel cooler produces the same reading, so establish which by watching whether the fault follows the level.
Typically an easy fix. Often worth checking before paying anyone to look.
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Codes where this is a likely explanation first, rather than merely a possible one.
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