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Carbon accumulating in the combustion chamber and glow pin of a fuel-fired auxiliary heater, until it can no longer ignite or burn cleanly. These heaters are small burners with a glow pin, a metering pump and a combustion chamber, and soot is what they produce when combustion is imperfect.

Short run cycles are what cause it, which is the useful part. A heater started and stopped before it reaches full temperature never burns off its own deposits, so the fouling accumulates every cycle. A heater used for ten minutes to clear a windscreen and switched off is being used in the way that fouls it fastest, and a periodic long run is genuinely preventative.

The fault presents as failure to start or as a shutdown shortly after ignition, with the module reporting a flame that was not detected or was lost. Check the exhaust and intake pipes for the heater as well — they are small-bore, they run under the vehicle, and a blocked or crushed one produces the same incomplete combustion that causes the soot in the first place.