Aftertreatment System

DEF | SCR & DPF & EGR Explained. What you need to know!

2003-2007 EGR Only

Exhaust Gas Recirculation (EGR)

2008-2010 DPF Only

Diesel Particulate Filter (DPF)
Exhaust Gas Recirculation (EGR)

2011-2018 DPF SCR

Diesel Particulate Filter (DPF)
Exhaust Gas Recirculation (EGR)
Selective catalytic reduction (SCR)


1. EGR explained
2. DPF explained
3. SCR explained
4. VGT explained
5. Test explained

Every truck manufacture has these components installed depending on the emission standard and the year produced.

1. EGR System

An EGR valve is mounted on the exhaust manifold. This valve regulates exhaust gases that go into the engine. A portion of the exhaust gases are re-directed from the EGR cooler, through various pipes and routes into the intake manifold. This has a temperature sensor, differential pressure to sense the flow of the air coming in.

This EGR system works with the VGT operation to create the right turbo drive pressures for the right amount of EGR Flow.

The 2003-2007 EGR systems on the older engine models do not have a VGT Turbo, so this EGR operation will work the same way but without regulating the turbocharger. Example: Volvo D12

2. DPF System

The exhaust aftertreatment system also consists of two parts designed to increase exhaust temperature for proper DPF regeneration. The aftertreatment Fuel Injector (AFI), mounted on the exit pipe from the turbocharger, injects diesel fuel into the exhaust stream during a regeneration cycle. When the fuel contacts the Diesel Oxidation Catalyst (DOC), a chemical reaction occurs, and the exhaust system temperature increases to a level that oxidizes the soot in the filter and reduces the emission out the tailpipe.

Under normal engine operation, the DPF collects soot and other particulate matter. These will be eventually oxidized using a regeneration process. The DPF filter can show sign of failure such as:

Temp Regeneration Lockout

Parked Regeneration Needed

High Soot Load Fault Codes (Level 2, 3, 4) 144%+

Active DPF Fault Codes

Excessive fuel mileage loss

Performance Loss

3. SCR / DEF System

The system will have the SCR system, DPF filter, EGR system and VGT turbocharger to regulate emissions. The SCR technology allows for passive regeneration. This occurs when the EGT at the DPF inlet and the NOx Ratio to soot ratio are high enough to oxidize the particulate matter. The process is slow and does not change the engine operation.

Most of the time you won’t even know that your truck is in the passive regen state. With this system forced stationary regeneration is only a backup when passive regeneration is insufficient. The soot ratio will increase and decrease depending on the operation of your system.

4. VGT Turbo Charger

The VGT regulates the charge pressure and drives the exhaust gas recirculation (EGR) system. The turbocharger speed is controlled by an electronic actuator and will adjust based on engine load conditions


Lowers emissions

Better fuel consumption

Increases power output

Common Problems

Active Faults for VGT

No pulling power / Derated

Costly repair

5. Testing

Diagnostic help can be found in various diagnostic tools such as Premium Tech Tool, JPro, Cummins Insite, Detroit Diesel DDDL.

  • Diesel Particulate Filter Service Regeneration
  • Exhaust Aftertreatment Diagnostics
  • Vairable Geometry Turbochager, Function Test
  • Exhaust Gas Recirculation, Function Test
  • SCR System Test

These tests can be conducted to see what issues you are having with your system. It is important to know the risks when performing any type of delete on your truck. We make sure to provide information and help guiding you to make a right decision to ensure reliability. It is important to check fault codes before diagnosing your problem as this can aid in fixing the issue.

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Send us a message with any inquiries or questions related to anything with your truck. We are here to help you any way we can.

Phone: 586.799.4375
Email: [email protected]