Emissions Control and Aftertreatment Systems
Modern diesel aftertreatment combines DOC (diesel oxidation catalyst), DPF (diesel particulate filter), SCR (selective catalytic reduction), and sometimes EGR. Know what each does: DOC oxidizes CO/HC and helps regenerate the DPF; DPF traps soot and is cleaned by active or passive regeneration; SCR uses DEF (urea) to reduce NOx; EGR lowers combustion temperature to cut NOx.
Key concepts: DPF differential pressure (restriction), soot load, regen types (passive, active, parked), DEF quality and level, SCR efficiency, and NOx sensor operation. Understand derate strategies when aftertreatment faults occur.
Traps: assuming black smoke is normal; it indicates over-fueling or DPF issues. DEF contamination or dilution causes SCR faults; never top with water. A DPF full of ash cannot be regenerated away - ash requires cleaning/replacement, unlike soot. EGR cooler leaks cause coolant loss and white smoke. Do not confuse DOC with DPF function.
Worked example: P2002 (DPF efficiency below threshold) with high differential pressure. Check for excessive soot load, then perform a parked regen; if pressure stays high after regen, suspect ash or a cracked filter and inspect/replace.
Rule of thumb: High DPF delta-P after a successful regen = ash or damage, not soot.