The flow
Exhaust → DOC (diesel oxidation catalyst, oxidizes HC/CO) → DPF (traps soot, burns it off during regen) → SCR (sprays DEF, reduces NOx to N2 + H2O). Each component has sensors before and after.
Modern diesel aftertreatment is four components working as a system. When one fails, the others suffer. Knowing the chain saves big money in diagnosis.
Shops that diagnose aftertreatment by replacing the loudest fault code rarely fix the real cause. Understanding the chain is the difference between $400 and $8,000 in parts.
Exhaust → DOC (diesel oxidation catalyst, oxidizes HC/CO) → DPF (traps soot, burns it off during regen) → SCR (sprays DEF, reduces NOx to N2 + H2O). Each component has sensors before and after.
DOC: face plug from over-fueling. DPF: ash plug from age, soot plug from upstream issues. SCR: catalyst poisoning from contaminated DEF. DEF dosing module: crystallization from leaks.
DPF traps and burns soot (particulates). SCR uses DEF to chemically reduce NOx. Different pollutants, different components, both required by EPA 2010+.