Modern diesels use two systems to clean their exhaust: the DPF (diesel particulate filter) traps soot, and the SCR system sprays DEF (diesel exhaust fluid) into the exhaust to cut NOx emissions. Most DPF problems come from a filter that can't burn off its soot because of short trips, idling or a fault elsewhere. Most DEF problems come from low or bad fluid, crystallized injectors, or failed sensors and heaters. Both can trigger warnings and eventually a power derate.
The DPF and regeneration
The DPF fills with soot as you drive. The engine cleans it by burning the soot off, which is called a regen:
- Passive regen: happens naturally at sustained highway speeds and exhaust temperatures
- Active regen: the engine raises exhaust temperature on purpose, often with extra fuel
- Parked or forced regen: started from the dash or with a scan tool when the truck can't regen on its own
Over time, ash that won't burn off builds up and the DPF needs professional cleaning or replacement.
Common causes of DPF trouble
- Lots of short trips, idling or low-speed work (it never gets hot enough)
- Interrupted regens, from shutting off the engine mid-regen
- Faulty sensors: differential pressure sensor, exhaust temperature sensors
- Engine problems that make too much soot: leaking injectors, EGR faults, boost leaks
- Ash loading at high mileage
DEF and the SCR system
DEF is a solution of high-purity urea and deionized water (about 32.5% urea). It's injected into the exhaust, where the SCR catalyst uses it to convert NOx into nitrogen and water.
Common DEF problems
- Low DEF: warnings, then derate if it runs out
- Contaminated or old DEF: quality sensor faults
- Crystallized DEF injector or lines: white crust, clogging
- Failed DEF heater in cold weather
- Failed NOx sensors, which are common fault points
- DEF pump or module failure
Never put DEF in the fuel tank. The DEF filler usually has a blue cap.
What a derate is
When the system sees an emissions fault it can't fix, it limits engine power or speed to protect the engine and force a repair. The longer a fault is ignored, the more severe the derate usually gets.
How a mobile diesel mechanic fixes it
Read the codes and live data with diesel scan software. See common OBD-II codes for how codes work generally
Check the sensors, wiring and DEF quality
Fix the root cause: injector, EGR, sensor or boost leak
Run a forced regen if the filter is loaded but healthy
Recommend cleaning or replacement if it's full of ash or damaged
Many aftertreatment repairs are done on site. Find a specialist under mobile diesel mechanics.
Winter makes it worse
DEF freezes at about 12°F (−11°C). The system has heaters for that, but failed heaters are a common winter fault. See diesel won't start in the cold.
Frequently asked questions
Can I delete the DPF or DEF system?
Removing or disabling emissions equipment is illegal under federal law for on-road vehicles and can bring heavy fines.
How often does a DPF need cleaning?
It depends on the engine, duty cycle and maintenance. Your engine maker publishes guidance.
Why does my truck regen so often?
Short trips, idling or a problem that's making excess soot. Frequent regens are worth diagnosing.
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