How to Troubleshoot After-Treatment Issues in Your Truck

Modern diesel trucks have after-treatment systems that reduce harmful emissions and meet federal EPA standards. These systems include components like the Diesel Particulate Filter (DPF), Selective Catalytic Reduction (SCR), Diesel Oxidation Catalyst (DOC), and Exhaust Gas Recirculation (EGR). 

While these technologies are vital for keeping the air clean, they can also be a major source of performance issues, downtime, and warning lights if they are not properly maintained or diagnosed.

If you’re dealing with check engine lights, frequent regens, or power loss, here’s how to identify and fix common after-treatment problems.

1. Identify Warning Signs Early

After-treatment system failures rarely happen suddenly. Your truck will often give you warning signs well before you’re parked on the side of the road in derate mode.

Common Symptoms of After-Treatment Issues:

• Check engine or MIL light illuminated
• DPF warning light flashing
• Loss of power or sluggish acceleration
• Frequent or failed regens
• Increased fuel consumption
• White or black smoke from the exhaust

Ignoring these signs can lead to forced derates, expensive roadside tows, or full system replacements.

2. Scan for Trouble Codes

When an after-treatment issue arises, the first step is to run a diagnostic scan. Most problems will trigger a fault code stored in the engine control module (ECM).

How to Perform a Scan:

• Use a dealer-level diagnostic tool or advanced scanner to access after-treatment modules.
• Record active and inactive fault codes, along with freeze frame data.
  
  
• Pay close attention to DPF efficiency codes, DEF pressure and quality codes, NOx sensor readings, and EGR flow rate data.

Having these codes on hand helps narrow down the root issue and avoid unnecessary part replacements.

3. Evaluate the Diesel Particulate Filter (DPF)

The DPF traps soot and carbon particles from the exhaust. Over time, it becomes clogged and must be cleaned through a process called regeneration.

DPF-Related Failures Include:

• Clogged or saturated filter
• Failed sensors (temperature, pressure differential)
• Frequent or incomplete regens
• Cracks in the ceramic filter

Troubleshooting Steps:

• Inspect DPF differential pressure readings; a high pressure = clogged filter.
• Confirm the regen process is completed. Failed regens may require forced regen or professional DPF cleaning.
• If readings are inconclusive, remove and inspect the DPF. Many filters need thermal or chemical cleaning at 150,000–250,000 miles.

4. Inspect the Selective Catalytic Reduction (SCR) System

The SCR system uses Diesel Exhaust Fluid (DEF) to reduce harmful NOx emissions. Malfunctions in this system can quickly lead to derate conditions and compliance issues.

SCR Problems Often Involve:

• DEF quality issues (contamination or expired fluid)
• Plugged DEF dosing valve or lines
• Crystallized DEF in the injector or tank
• Failed NOx sensors or catalyst inefficiency

Troubleshooting Tips:

• Check DEF level, age, and quality—replace fluid if contaminated.
• Inspect injector for crystallization—clean or replace as needed.
• Run a SCR system test to verify catalyst efficiency.
• Replace NOx sensors if readings are outside manufacturer spec.

Never run the truck with poor-quality DEF. It can lead to permanent SCR damage.

5. Diagnose EGR and DOC-Related Issues

The EGR system recirculates exhaust gas to reduce combustion temperatures and NOx emissions. The DOC converts CO and hydrocarbons into less harmful gases. Problems in either can impact DPF and SCR performance.

Watch for These Signs:

• Excess soot or carbon buildup in intake or EGR valve
• Sticky or stuck EGR valves
• Slow or no response from the EGR actuator
• Poor DOC catalyst performance

What to Check:

• Inspect the EGR valve and cooler for blockage or leakage.
• Clean or replace components with excessive carbon buildup.
• Test the DOC for proper temperature rise during regen—low rise may indicate catalyst degradation.

Failing to address EGR or DOC issues can result in a cascading effect—clogging the DPF and throwing off SCR performance.

6. Perform a Forced Regeneration (If Safe to Do So)

If the DPF is clogged and passive regens are failing, a forced regeneration may be necessary to burn off excess soot and restore proper flow.

How to Safely Perform a Forced Regen:

• Ensure the truck is in a safe, open, and ventilated area.
• Connect your scan tool and initiate the manual regen cycle.
• Monitor temperature, backpressure, and engine load throughout the process.

Note: If the regen fails or temperatures don’t rise properly, the DPF may require removal and professional cleaning.

Final Thoughts

After-treatment systems are complex, but they don’t have to be a mystery. By understanding how each component works—and how to spot signs of failure—you can stay ahead of costly repairs and derates.