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The exhaust outlet of a fuel-fired auxiliary heater blocked, so combustion gases cannot escape. The heater is a small burner with its own exhaust pipe running under the vehicle, and that pipe is narrow enough to be closed by road debris, packed snow, or corrosion.

A blocked outlet gives incomplete combustion. The flame is smothered, the unit shuts down on a flame or over-temperature fault, and every failed cycle deposits more soot in the burner — so the blockage causes the fouling that then becomes a second fault.

It is also a safety consideration rather than only a heater fault, since the exhaust must vent clear of the vehicle. Check the outlet and the intake together — both are small-bore, both run under the vehicle, and a heater parked in deep snow can have both blocked at once. Look for soot staining around the outlet, which indicates the unit has been running badly rather than not at all.