How does Carbon build up on the inside of your engine?

Over time, carbon accumulates in various parts of the engine. One place, in particular is the EGR tube. Once carbon builds to the point that exhaust cannot flow through the tube, the DPFE detects there is a problem and illuminates the check engine light. Some EGR passages are easily accessible while others are not, and require a great deal of work to clean.

Engine Carbon Build up

Diesel Particulate Filter