The Complex World of Semi Truck Aftertreatment
Today’s semi trucks run under the strictest emissions rules in trucking history. DPF, DOC, SCR, DEF, and EGR systems have cleaned up exhaust dramatically, but they have also become the number one source of derates, roadside calls, and five figure repair bills.
Ask any fleet what puts their trucks in derate the most and you will hear the same list. DPF problems. SCR efficiency codes. NOx sensor failures. DEF system malfunctions. EGR cooler or valve issues. Even well maintained trucks eventually experience these problems, especially once they pass the 300,000 to 450,000 mile range.
This guide breaks down the entire system in plain language. What each component does, why it fails, which platforms are known for certain issues, how to spot early warning signs, what typical repairs cost, how to reduce failure frequency, and how extended coverage, including TruckProtect options, can help stabilize costs when the emissions light comes on.
Diesel Particulate Filter Failures
Across major engines, Volvo D13, Detroit DD15, Paccar MX13, Cummins X15, International A26, DPF problems are the most common aftertreatment issue. The DPF is a ceramic filter that traps soot from combustion and then burns it off during regeneration.
DPFs fail primarily because they get overloaded with soot and ash. Excessive soot load, incomplete passive regens, interrupted active regens, clogged inlet or outlet ports, ash accumulation at end of life, thermal stress and cracking, low exhaust temperatures in winter or light duty, and high idle time all contribute.
Warning signs include high soot load readings, more frequent regens, reduced power, poor fuel mileage, elevated exhaust gas temperatures, and rough idle. If you ignore those signs, the truck will eventually derate or force a parked regen that may not complete.
Cleaning a DPF typically costs 300 to 600 dollars. Full replacement runs around 3,000 to 6,000 dollars. If the DOC is replaced at the same time, a DPF and DOC combo can easily cost 5,000 to 10,000 dollars. Many TruckProtect plans include DPF coverage because this is such a common and expensive failure point.
Diesel Oxidation Catalyst Failures
The DOC sits upstream of the DPF. Its job is to oxidize unburned hydrocarbons and create the heat needed for regeneration. If the DOC is not working properly, the DPF will struggle to burn soot, even if everything else looks fine.
DOCs fail from overheating, thermal cracking, contamination from fuel, oil, or coolant, poor regen cycles, long idle time, and cold weather operation that never lets the catalyst reach full temperature.
Warning signs often show up as regen failures, decreased exhaust temperature during regen attempts, DOC or DPF efficiency codes, and DPF problems that come back soon after cleaning. In many cases, a fleet keeps cleaning or replacing DPFs while the real issue is an aging or damaged DOC.
DOC replacement usually costs between 2,000 and 5,000 dollars. On some integrated systems, it is part of a larger aftertreatment assembly, which increases the total bill.
Selective Catalytic Reduction Failures
The SCR system is responsible for reducing NOx emissions by injecting DEF into the exhaust stream and passing it over a catalyst. When it works, NOx levels drop sharply. When it fails, you get codes, derates, and angry drivers.
SCR units tend to fail more frequently after 400,000 miles. Common causes include DEF crystallization in lines and injectors, poor DEF quality, injector failures, NOx sensor problems, catalyst poisoning from contaminants, and thermal cracking from repeated heat cycles.
Warning signs include SCR efficiency codes, NOx out of range codes, a drop in fuel economy, and derates triggered by NOx control failures. Sometimes the SCR brick itself is damaged. Other times, bad data from sensors or DEF issues cause the system to behave as if the catalyst has failed.
SCR replacement typically costs 4,000 to 10,000 dollars. On some Detroit DD15 OneBox setups, the total can run 7,000 to 12,000 dollars. Because of these numbers, SCR coverage is a major value point in many TruckProtect plans.
DEF System Failures
The DEF system includes the DEF pump, doser or injector, DEF header and level or quality sensor, lines, and tank heater. These parts do not just fail on their own, they often trigger or accelerate SCR failures.
DEF systems fail due to contaminated DEF fluid, pump wear, bad DEF quality sensors, frozen DEF in cold weather, clogged lines, and corrosion in connectors and fittings. Cheap or improperly stored DEF is a major contributor.
Symptoms include DEF level or quality sensor faults, improper dosing, derate warnings tied to DEF issues, and SCR efficiency codes that do not clear even after other repairs. If the DEF system cannot deliver the right amount of fluid at the right time, the SCR cannot do its job.
Typical costs, DEF pumps run 1,200 to 3,500 dollars, DEF headers 800 to 2,500 dollars, DEF injectors 400 to 900 dollars. When you add labor and diagnostics, a simple DEF issue can become a multi thousand dollar visit.
NOx And Other Sensor Failures
Sensors are the weak link in modern emissions systems. They are small, relatively inexpensive parts that live in a brutal environment of heat, vibration, moisture, and contamination. When they fail or drift out of spec, the entire system can react badly.
Common failures include NOx sensors, DPF pressure sensors, DOC temperature sensors, SCR temperature sensors, exhaust gas temperature sensors, and differential pressure sensors. Industry data suggests that sensors are responsible for 40 to 60 percent of derates.
Causes include vibration, moisture intrusion, heat cycling, wiring harness corrosion, DEF crystallization on connectors, and dust or dirt contamination. A sensor may not be completely dead, but if it sends bad data, the ECM will still make bad decisions.
Costs vary. NOx sensors usually run 400 to 1,000 dollars each. EGT sensors are often 100 to 400 dollars. Pressure sensors are typically 250 to 500 dollars. Many TruckProtect emissions tiers include sensor coverage because these small parts create big headaches.
EGR Valve And EGR Cooler Failures
The EGR system recirculates a portion of exhaust gas back into the intake to reduce NOx formation. It is effective for emissions, but it introduces a lot of soot and heat into parts of the engine that were not originally designed for it.
EGR systems fail due to soot buildup, coolant side cracking in the cooler, valve sticking, coolant contamination, high exhaust temperatures, and poor maintenance. Over time, soot narrows passages, valves gum up, and coolers crack internally.
Warning signs include white smoke from coolant entering the exhaust, loss of power, poor fuel economy, clogged EGR passages, and higher than normal engine temperatures. Sometimes the first clue is a code for EGR flow that will not clear.
An EGR valve typically costs 600 to 1,500 dollars. An EGR cooler replacement usually runs 2,000 to 5,000 dollars. These repairs are common enough that many extended coverage plans, including TruckProtect, specifically call out EGR components.
Why Aftertreatment Failures Increase After 300,000 To 450,000 Miles
There is a clear pattern in high mileage trucks. Once you cross into the 300,000 to 450,000 mile band, aftertreatment problems become more frequent and more expensive.
As engines age, exhaust soot increases because combustion is less efficient. Emissions systems clog as ash accumulates in the DPF. Sensors degrade from years of heat and vibration. DOC and SCR catalysts lose efficiency, just like brake linings or tires wear down. DEF systems see more crystallization and chemical breakdown in lines, injectors, heaters, and pumps. EGR coolers fatigue and are more likely to crack or leak.
None of this is a surprise from a physics standpoint. These parts are doing hard work in a harsh environment, and they have a finite life. The key is recognizing that once a truck passes this mileage range, your maintenance and budget strategy needs to adjust.
Brand Specific Aftertreatment Notes
Different platforms have different patterns. Adding these insights helps fleets diagnose and plan.
Cummins X15 engines are known for frequent NOx sensor failures, EGR cooler cracking, and DEF issues in cold weather if DEF handling is not ideal.
Detroit DD15 engines often see OneBox related failures, including SCR cracking from heat stress and integrated aftertreatment issues that require larger assemblies to be replaced.
Volvo D13 platforms can experience EGR valve sticking, DPF and DOC combo degradation, and injector issues that affect regen quality and soot load.
Paccar MX13 engines have a history of fuel system contamination problems, turbo actuator issues, and high soot load in short haul or regional duty cycles.
International A26 engines have seen EGR cooler cracking, sensor failures, and DEF header issues in some fleets.
These patterns are not universal, but they show up often enough that many shops and fleets recognize them immediately.
How To Prevent Aftertreatment Failures
You cannot eliminate every failure, but you can lower the odds and push big problems further into the future.
Reducing idle time is one of the most effective steps. Idle is the number one cause of soot loading. Training drivers and using idle shutdown policies where appropriate can make a big difference.
Increasing highway run time helps. Regular highway miles at proper load and temperature support passive regen and keep the DPF healthier.
Performing DPF cleanings on a schedule is critical. A common rule of thumb is around 300,000 miles or 4,500 engine hours, but duty cycle matters. Short haul or idle heavy trucks may need cleaning sooner.
Using high quality DEF only, stored correctly, prevents a long list of SCR and pump failures. Cheap or contaminated DEF is a false economy.
Replacing sensors at the first warning, instead of clearing codes and hoping for the best, reduces the risk of secondary damage.
Fixing coolant leaks immediately protects DOC and SCR units from contamination. Coolant in the exhaust will ruin catalysts quickly.
Keeping the engine tuned and fuel system healthy reduces soot production. Poor tune equals high soot, and high soot equals DPF death.
Where Extended Warranty Coverage Helps
Aftertreatment repairs are expensive, unpredictable, downtime heavy, and often repetitive. A truck might need a DPF this year, an SCR next year, and a set of sensors in between.
TruckProtect plans are designed to address that reality. Coverage often includes DPF, DOC, SCR, DEF pumps, key sensors, EGR valves, and EGR coolers, depending on the tier. Coverage does not change the physics of soot or heat, but it does change how those failures hit your balance sheet.
Instead of every event being a surprise five figure bill, fleets can turn some of that risk into a predictable cost. That is especially valuable for high mile or used semi trucks where OEM coverage is gone but the truck still has years of work left.
Conclusion, Semi Truck Aftertreatment Systems Are Complex, But You Can Manage The Risk
Modern aftertreatment is complicated, but it is not mysterious. DPF failures come from soot, ash, and regen problems. DOC units crack from heat and contamination. SCR failures are often tied to DEF and NOx issues. EGR problems come from soot and coolant contamination. Sensors fail constantly because of heat and vibration. High mileage accelerates every one of these patterns.
You cannot stop every breakdown, but you can understand what is really happening, adjust maintenance, train drivers, and budget for the reality of modern emissions systems. TruckClub’s role is to give you that clarity. TruckProtect’s role is to provide protection when critical aftertreatment components inevitably fail in the real world.
