Diesel Aftertreatment: DPF, DEF & SCR (ASE A9)

1. What does the Diesel Particulate Filter (DPF) actually trap, and how does it get rid of it?

• A. It traps soot (particulate matter) and burns it off as CO2 during regeneration ✓
• B. It traps NOx gases and neutralizes them with a chemical wash
• C. It traps unburned diesel fuel and drains it back to the tank
• D. It traps DEF crystals and dissolves them with engine heat

Answer: A — It traps soot (particulate matter) and burns it off as CO2 during regeneration. The DPF is a honeycomb filter that physically catches soot (carbon particulate) from the exhaust. Once it fills, the truck raises exhaust temperature high enough to oxidize that trapped soot into CO2 — that process is called regeneration. NOx is handled separately by SCR/DEF, not the DPF. The takeaway: the DPF must periodically burn off what it collects, or it clogs.

2. A driver only takes 2-mile trips around town and the truck has never gotten warm. Why is this the worst-case scenario for the DPF?

• A. Short trips overheat the DPF and crack the substrate
• B. The exhaust never gets hot enough or runs long enough to complete a regeneration, so soot keeps building up ✓
• C. Short trips flood the DPF with DEF, contaminating it
• D. Short trips force constant active regens that wear out the filter prematurely

Answer: B — The exhaust never gets hot enough or runs long enough to complete a regeneration, so soot keeps building up. Regeneration needs sustained high exhaust temperature and time. Constant short, cold trips mean a regen either never starts or gets interrupted before it finishes, so soot accumulates with nowhere to go. Eventually you get a clogged DPF, a warning light, and possibly a derate. The takeaway: diesels need occasional sustained highway-speed driving to stay healthy.

3. What is the difference between a PASSIVE regen and an ACTIVE regen?

• A. Passive is done by a technician with a scan tool; active happens while driving
• B. Passive burns soot; active burns DEF crystals
• C. Passive happens automatically at normal high exhaust temps; active is triggered by the ECU injecting extra fuel to raise temperature on purpose ✓
• D. Passive uses DEF; active uses the EGR valve

Answer: C — Passive happens automatically at normal high exhaust temps; active is triggered by the ECU injecting extra fuel to raise temperature on purpose. Passive regen happens on its own when the exhaust is naturally hot enough (sustained highway load) to oxidize soot — the driver never notices. Active regen is when the ECU deliberately raises exhaust temperature, typically by injecting extra fuel (post-injection or a dosing injector), because passive conditions weren't met. Active regen is the computer compensating for driving that didn't keep things hot enough.

4. A PARKED (forced/stationary) regen is required. What does that tell you about the situation?

• A. The DPF is brand new and needs to be initialized
• B. The DEF tank is empty and must be refilled before driving
• C. The SCR catalyst has failed and must be replaced
• D. Soot loading got high enough that normal driving regens couldn't keep up, and the truck now needs to sit stationary at high RPM to burn it down ✓

Answer: D — Soot loading got high enough that normal driving regens couldn't keep up, and the truck now needs to sit stationary at high RPM to burn it down. A parked regen is the last automatic line of defense: soot loading climbed too high (missed/interrupted active regens), so the truck must sit, hold elevated RPM, and run a long high-temperature burn. It can take 20-40+ minutes and ties up the vehicle. The takeaway: needing parked regens is a symptom of a driving pattern or fault that's preventing normal regens — fix the root cause, not just the soot.

5. Diesel Exhaust Fluid (DEF) is specifically what?

• A. A 32.5% urea / 67.5% deionized water solution dosed into the exhaust ✓
• B. A diesel fuel additive that boosts cetane
• C. A 50/50 mix of antifreeze and water for the SCR cooler
• D. Pure ammonia injected into the DPF

Answer: A — A 32.5% urea / 67.5% deionized water solution dosed into the exhaust. DEF is a precise 32.5% high-purity urea and 67.5% deionized water blend (the standardized AdBlue spec). It is NOT a fuel additive and never goes in the fuel tank. In the hot exhaust it breaks down into ammonia, which is what the SCR catalyst uses. The takeaway: DEF goes in its own separate (usually blue-capped) tank, never in the diesel.

6. What does the SCR (Selective Catalytic Reduction) system actually accomplish using DEF?

• A. It traps soot so the DPF doesn't have to
• B. It converts harmful NOx into harmless nitrogen (N2) and water vapor ✓
• C. It cools the exhaust to protect the turbo
• D. It converts CO2 into oxygen

Answer: B — It converts harmful NOx into harmless nitrogen (N2) and water vapor. SCR is the NOx killer. DEF is sprayed into the hot exhaust, decomposes to ammonia, and over the SCR catalyst that ammonia reacts with NOx to produce plain nitrogen gas and water — both harmless and already the main components of air. That's why running out of DEF defeats emissions control and triggers protections. The takeaway: DPF handles soot, SCR (with DEF) handles NOx — two different jobs.

7. The DEF tank runs empty and the driver keeps ignoring the warnings. What is the legally-mandated consequence built into virtually every modern diesel?

• A. Nothing — the truck runs normally but pollutes more
• B. The DPF is bypassed automatically to compensate
• C. The engine enters a power derate / limp mode and may eventually limit speed to a crawl until DEF is added ✓
• D. The fuel injectors shut off to prevent damage

Answer: C — The engine enters a power derate / limp mode and may eventually limit speed to a crawl until DEF is added. Emissions regulations require the truck to force the issue. As DEF gets low you get escalating warnings, then a power derate, and if still ignored, a severe speed limit (often ~5 mph) at the next restart. It is not optional and not a malfunction — it's designed to make running without DEF impractical. The takeaway: keep DEF topped off; an empty tank can strand you with a crawling truck.

8. Code P203F or P204F (reductant/DEF level too low or performance) is set. What's the correct first move?

• A. Immediately replace the DEF injector
• B. Delete the SCR system
• C. Force a parked DPF regen
• D. Check the actual DEF level and quality first, and verify the DEF level sensor is reading correctly ✓

Answer: D — Check the actual DEF level and quality first, and verify the DEF level sensor is reading correctly. Reductant level/performance codes most often come down to the obvious: low DEF, or a level sensor misreading. Before condemning the injector or pump, confirm the tank is genuinely full of good, in-spec DEF and that the sensor/float matches reality. Many of these are cheap fixes (fill the tank, replace a faulty level sensor). The takeaway: verify level and quality before throwing parts at it.

9. Code P20EE indicates 'SCR NOx Catalyst Efficiency Below Threshold.' What is it really telling you?

• A. The SCR system isn't reducing NOx as much as it should — measured NOx out is too high for the DEF being dosed ✓
• B. The DPF is physically cracked
• C. The DEF tank heater has failed
• D. The EGR valve is stuck open

Answer: A — The SCR system isn't reducing NOx as much as it should — measured NOx out is too high for the DEF being dosed. P20EE means the SCR catalyst's measured NOx-reduction efficiency fell below the calibrated limit — the downstream NOx sensor is still seeing too much NOx. Causes include contaminated or watered-down DEF, a failing DEF dosing/injector, a poisoned or aged SCR catalyst, NOx sensor faults, or exhaust leaks upstream. The takeaway: P20EE is an efficiency complaint about the whole SCR loop, not proof the catalyst brick itself is dead.

10. Code P2002 reads 'Diesel Particulate Filter Efficiency Below Threshold.' Before assuming the DPF brick is bad, what should you rule out?

• A. Low DEF level
• B. The DPF differential-pressure sensor and its hoses — a faulty sensor or plugged/leaking lines can mimic a bad filter ✓
• C. A weak alternator
• D. Wrong-viscosity engine oil

Answer: B — The DPF differential-pressure sensor and its hoses — a faulty sensor or plugged/leaking lines can mimic a bad filter. The ECU judges DPF condition largely from the differential (delta) pressure across the filter. A failed delta-P sensor, or hoses that are cracked, plugged with soot, or swapped, will give bad readings and can set P2002 even with a serviceable filter. Always verify the sensor and its plumbing (and check for a cracked/melted substrate or upstream issues) before buying an expensive DPF. The takeaway: confirm the pressure sensor is honest before condemning the filter.

11. While the DPF traps soot and SCR handles NOx, what is the main role of the EGR (Exhaust Gas Recirculation) system in this emissions chain?

• A. It injects DEF into the exhaust
• B. It physically filters soot before the DPF
• C. It recirculates a portion of exhaust back into the intake to lower combustion temperature, which reduces how much NOx is formed in the first place ✓
• D. It stores excess DEF for cold-weather starts

Answer: C — It recirculates a portion of exhaust back into the intake to lower combustion temperature, which reduces how much NOx is formed in the first place. EGR is upstream prevention: by routing cooled exhaust gas back into the cylinders, it lowers peak combustion temperature, and since NOx forms at high temperature, less NOx is created to begin with. SCR then cleans up what remains. The trade-off is that EGR can increase soot, which the DPF must handle. The takeaway: EGR reduces NOx at the source, SCR reduces it after the fact — they work together.

12. What does the DOC (Diesel Oxidation Catalyst), which sits ahead of the DPF, contribute during an active regeneration?

• A. It stores DEF until the SCR needs it
• B. It traps the soot so the DPF stays clean
• C. It converts NOx into nitrogen by itself
• D. It oxidizes hydrocarbons/CO and the extra dosed fuel, generating the heat needed to raise exhaust temperature and ignite the soot in the DPF ✓

Answer: D — It oxidizes hydrocarbons/CO and the extra dosed fuel, generating the heat needed to raise exhaust temperature and ignite the soot in the DPF. The DOC is an oxidation catalyst that burns off hydrocarbons and CO and, critically during active regen, reacts with the extra fuel dosed into the exhaust to create the high temperature that then burns the soot loaded in the downstream DPF. A degraded DOC can prevent regens from reaching temperature, causing repeated incomplete regens and DPF plugging. The takeaway: the DOC is the 'lighter' that lets the DPF burn its soot.

13. A DPF traps soot, which regeneration burns off. But the filter still eventually loads up over many miles. Why?

• A. Incombustible ash (mostly from oil additives and engine wear metals) accumulates and does not burn off in regen ✓
• B. Soot permanently bonds to the filter and never burns
• C. DEF crystallizes inside the DPF
• D. The filter absorbs diesel fuel

Answer: A — Incombustible ash (mostly from oil additives and engine wear metals) accumulates and does not burn off in regen. Regeneration burns the carbon soot to gas, but it cannot remove ash — the incombustible mineral residue left mainly by engine oil additives and wear metals. Ash slowly fills the DPF over the long term (often 100k+ miles), eventually requiring off-vehicle cleaning or DPF replacement even on a healthy engine. Soot is managed by regen; ash is the long-term wear-out item. DEF doesn't enter the DPF, and the filter doesn't absorb fuel.

14. How does the ECM estimate how loaded the DPF is so it knows when to command a regen?

• A. It weighs the DPF in real time
• B. It uses a differential-pressure sensor reading the pressure drop across the DPF, combined with a soot model ✓
• C. It counts the number of cold starts
• D. It measures DEF tank level

Answer: B — It uses a differential-pressure sensor reading the pressure drop across the DPF, combined with a soot model. A differential-pressure (delta-P) sensor measures the exhaust pressure difference between the inlet and outlet of the DPF; a loaded filter restricts flow and raises that pressure drop. The ECM combines this with a calculated soot model (fuel use, driving conditions) to decide when to regenerate. A plugged delta-P hose or bad sensor can cause false or missed regens. The DPF isn't weighed, and cold-start counts and DEF level don't measure soot loading.

15. What happens to DEF (diesel exhaust fluid) in very cold weather, and how do vehicles handle it?

• A. It explodes if it freezes
• B. It turns into diesel fuel
• C. It freezes at about 12°F (-11°C); tanks and lines have heaters, and properly specified DEF is undamaged by freeze/thaw ✓
• D. It becomes permanently unusable after freezing once

Answer: C — It freezes at about 12°F (-11°C); tanks and lines have heaters, and properly specified DEF is undamaged by freeze/thaw. DEF (32.5% urea / 67.5% deionized water) freezes at roughly 12°F (-11°C). The standard accounts for this: properly specified DEF freezes and thaws without degrading, and vehicles use DEF tank and line heaters so the system works in winter. Freezing is normal and not damaging to correct-spec fluid. It does not explode, convert to fuel, or become permanently ruined by a single freeze.

16. DEF must meet the ISO 22241 specification. Why does using off-spec or contaminated DEF matter?

• A. Any urea-and-water mix works equally well
• B. Contaminated DEF improves NOx conversion
• C. DEF purity only affects fuel economy
• D. Wrong concentration or contamination (water, diesel, dirt) damages the SCR catalyst and triggers DEF-quality codes and derates ✓

Answer: D — Wrong concentration or contamination (water, diesel, dirt) damages the SCR catalyst and triggers DEF-quality codes and derates. SCR catalysts are sensitive to DEF purity and concentration. Off-spec fluid — diluted, over-concentrated, or contaminated with diesel, water, or dirt — can poison or foul the catalyst, cause poor NOx conversion, plug the dosing system, and set DEF-quality codes that lead to power derates. That's why only ISO 22241 / API-certified DEF should be used. Not every urea/water mix is acceptable, contamination never improves conversion, and the impact is emissions/derate, not just economy.

17. A diesel sets reductant-system codes and you find hard white crystalline deposits around the DEF dosing injector and decomposition area. What does this indicate?

• A. Urea crystallization/deposits — often from low exhaust temperature, poor dosing, or a faulty injector — clogging the system ✓
• B. Normal salt buildup that needs no attention
• C. Coolant leaking from the radiator
• D. Excess engine oil in the exhaust

Answer: A — Urea crystallization/deposits — often from low exhaust temperature, poor dosing, or a faulty injector — clogging the system. When DEF doesn't fully decompose (low exhaust temperature, a dribbling/faulty dosing injector, or interrupted flow), the urea can form hard white crystalline deposits in the dosing injector, decomposition tube, or mixer, restricting flow and hurting SCR performance. The fix addresses the dosing fault and clears the deposits. It isn't harmless salt, coolant, or oil — it's urea crystallization, a known SCR failure mode.

18. What is the role of the NOx sensors in an SCR system, and why do they matter for diagnosis?

• A. They measure soot load in the DPF
• B. An upstream and a downstream NOx sensor let the ECM measure conversion efficiency; a failed NOx sensor is a common cause of SCR-efficiency codes and false derates ✓
• C. They control fuel injection timing only
• D. They sense DEF tank temperature

Answer: B — An upstream and a downstream NOx sensor let the ECM measure conversion efficiency; a failed NOx sensor is a common cause of SCR-efficiency codes and false derates. SCR systems typically use a NOx sensor before the catalyst (engine-out) and one after it; the ECM compares the two to calculate how much NOx the SCR is removing. Because the system trusts these readings, a failed or drifting NOx sensor is one of the most common causes of SCR-efficiency codes (like P20EE) and unwarranted derates — so verify the sensors before condemning the catalyst. They don't measure DPF soot, set injection timing, or read DEF temperature.

19. What is 'ammonia slip' in an SCR system?

• A. DEF leaking onto the ground
• B. Soot escaping the DPF
• C. Unreacted ammonia passing downstream when DEF is over-dosed or the catalyst can't convert it all ✓
• D. Coolant entering the SCR

Answer: C — Unreacted ammonia passing downstream when DEF is over-dosed or the catalyst can't convert it all. SCR converts NOx using ammonia released from DEF. If too much DEF is dosed, or the catalyst is cold/degraded, some ammonia passes through unreacted — 'ammonia slip' (a secondary emission with a sharp odor). Systems carefully control dosing and may add a clean-up/ammonia-slip catalyst downstream to oxidize the excess. It's not a DEF leak, soot escape, or coolant intrusion — it's surplus ammonia getting past the catalyst.

20. Why can't an SCR system dose DEF effectively when the exhaust is too cold (for example, extended idling)?

• A. Cold DEF is too thick to spray
• B. The NOx sensors only work below freezing
• C. DEF only works at wide-open throttle
• D. Urea needs a minimum exhaust temperature (roughly 200°C) to decompose into the ammonia SCR uses, and to avoid forming deposits ✓

Answer: D — Urea needs a minimum exhaust temperature (roughly 200°C) to decompose into the ammonia SCR uses, and to avoid forming deposits. DEF must thermally decompose (hydrolyze) into ammonia to drive the SCR reaction, which requires sufficient exhaust temperature — roughly 200°C and up. When the exhaust is too cold (long idling, very light load), the system limits or stops DEF dosing both because conversion is poor and because dosing into cold exhaust forms urea deposits. That's why prolonged idling can hurt diesel emissions performance. It's about decomposition temperature, not DEF viscosity, sensor range, or throttle position.

21. A variable-geometry turbocharger (VGT) on a modern diesel does what, and what's a common failure?

• A. Its movable vanes adjust boost across the RPM range and can raise exhaust temperature to aid regen; soot/carbon can stick the vanes, causing low power and boost codes ✓
• B. It only spins at idle
• C. It replaces the need for an intercooler
• D. It stores DEF

Answer: A — Its movable vanes adjust boost across the RPM range and can raise exhaust temperature to aid regen; soot/carbon can stick the vanes, causing low power and boost codes. A VGT uses movable vanes to optimize boost from low to high RPM and can be commanded toward a more closed position to raise exhaust backpressure/temperature to help DPF regeneration. Soot and carbon can seize those vanes (or the actuator can fail), causing low power, over/underboost, and turbo/boost-control codes. Cleaning or replacing restores function. It doesn't only spin at idle, replace the intercooler, or store DEF.

22. On a diesel, an EGR cooler develops an internal leak. What is a likely symptom?

• A. Higher fuel economy
• B. Coolant loss with white smoke from the exhaust (coolant being drawn into the intake/exhaust), possibly a sweet smell ✓
• C. A louder horn
• D. Brighter headlights

Answer: B — Coolant loss with white smoke from the exhaust (coolant being drawn into the intake/exhaust), possibly a sweet smell. EGR coolers use engine coolant to cool recirculated exhaust gas. An internal crack lets coolant leak into the EGR/intake path, where it's burned — producing white smoke, gradual coolant loss (often with no external leak), a sweet exhaust smell, and in severe cases risk of hydrolock. It's a common, expensive diesel failure to check when a diesel loses coolant with white smoke. It has nothing to do with fuel economy, the horn, or lighting.

23. Black smoke from a diesel exhaust most directly indicates:

• A. Burning engine oil
• B. Coolant in the combustion chamber
• C. Incomplete combustion — over-fueling or insufficient air (restricted intake, boost leak, faulty injectors, or a failing turbo) ✓
• D. A perfectly tuned engine

Answer: C — Incomplete combustion — over-fueling or insufficient air (restricted intake, boost leak, faulty injectors, or a failing turbo). Black diesel smoke is unburned/partially burned fuel — too much fuel for the available air. Causes include a clogged air filter or boost (charge-air) leak, over-fueling or leaking injectors, or a failing turbo not making enough air. (Blue/gray smoke = burning oil; white smoke = unburned fuel from cold/low compression/timing, or coolant.) Reading smoke color quickly narrows the diagnosis. Black is the air/fuel-imbalance color, not oil, coolant, or a healthy engine.

24. A turbo-diesel is down on power with low boost and possibly black smoke, but the turbo itself checks out. What should you suspect?

• A. A weak windshield washer pump
• B. Worn brake pads
• C. A clogged cabin air filter
• D. A charge-air (boost) leak — a split intercooler hose, cracked intercooler, or loose clamp letting pressurized air escape ✓

Answer: D — A charge-air (boost) leak — a split intercooler hose, cracked intercooler, or loose clamp letting pressurized air escape. If the turbo is good but boost is low, pressurized air is escaping before the engine can use it — a split charge-air (intercooler) hose, a cracked intercooler, or a loose clamp. The engine loses power and can smoke black from the air shortfall. A charge-air pressure (smoke) test of the intake/intercooler system finds it. The washer pump, brakes, and cabin filter have nothing to do with boost.

25. Why do modern diesels route crankcase blow-by through a closed crankcase ventilation (CCV) / oil-separator filter, and what happens when it clogs?

• A. It vents and separates oil from blow-by gases; a clogged CCV builds crankcase pressure, pushing oil past seals and causing leaks ✓
• B. It stores DEF for the SCR system
• C. It cools the transmission
• D. It charges the battery

Answer: A — It vents and separates oil from blow-by gases; a clogged CCV builds crankcase pressure, pushing oil past seals and causing leaks. Closed crankcase ventilation routes piston blow-by through an oil separator/filter and back into the intake (instead of venting to atmosphere), keeping oil out of the intake while relieving crankcase pressure. When the CCV filter clogs, crankcase pressure builds and forces oil past seals and the dipstick, causing leaks and oil consumption. It's a maintenance item on many diesels. It doesn't store DEF, cool the transmission, or charge anything.

26. A diesel is regenerating much more often than normal. What does increasing regen frequency usually mean?

• A. It is a normal sign the engine is healthy
• B. The engine is producing excess soot (leaking injectors, stuck-open EGR, turbo/boost fault) or the DPF is nearing its ash limit — find the root cause ✓
• C. The DEF tank is full
• D. The battery is overcharged

Answer: B — The engine is producing excess soot (leaking injectors, stuck-open EGR, turbo/boost fault) or the DPF is nearing its ash limit — find the root cause. Regens that come more and more frequently mean the DPF is loading with soot faster than it should — commonly from dribbling/leaking injectors, a stuck-open EGR, a turbo/boost problem, or excessive idling, or the DPF is approaching its ash capacity. Just letting it keep regenerating wastes fuel and stresses the system; you investigate why soot production is high. It's not a sign of health, and DEF level/battery charge don't drive regen frequency.

27. During an active regeneration, how is the high heat needed to burn the DPF soot actually generated?

• A. The DPF has an electric heating element that glows red
• B. DEF is sprayed onto the DPF and ignited
• C. Extra fuel (late post-injection or a dedicated dosing injector) is oxidized in the DOC, raising exhaust temperature enough to burn the soot ✓
• D. The turbo is shut off to trap heat

Answer: C — Extra fuel (late post-injection or a dedicated dosing injector) is oxidized in the DOC, raising exhaust temperature enough to burn the soot. Active regen raises exhaust temperature by introducing extra fuel — through late/post in-cylinder injection or a dedicated downstream hydrocarbon doser — which is oxidized (burned) in the diesel oxidation catalyst (DOC) ahead of the DPF. That exothermic reaction pushes exhaust temperature high enough (roughly 550-600°C+) to combust the trapped soot. Most light-vehicle DPFs are not electrically heated, DEF is for SCR (not DPF burning), and the turbo isn't shut off.

28. A driver has ignored escalating DPF warnings and the soot load is now very high. Why can't the system just run a normal automatic regen?

• A. Automatic regens are disabled above freezing
• B. The DEF must be drained first
• C. The battery must be replaced before any regen
• D. Too much soot makes an automatic active regen unsafe (risk of uncontrolled high temperatures); it now needs a parked/forced regen with a tool or DPF service ✓

Answer: D — Too much soot makes an automatic active regen unsafe (risk of uncontrolled high temperatures); it now needs a parked/forced regen with a tool or DPF service. Automatic active regens are calibrated for a normal soot range. If the DPF gets overloaded (warnings ignored), burning that much soot at once could spike temperatures and damage the DPF, so the ECM blocks the automatic regen and requires a controlled parked/forced regen with a scan tool — or, if it's too far gone, DPF cleaning/replacement. Pushing a badly clogged DPF risks melting it or triggering a derate. Temperature thresholds, DEF, and the battery aren't the gating factors here.

29. Regulatory DEF/SCR 'inducement' on modern diesels works how when DEF runs low or quality fails?

• A. A stepped escalation: dashboard warnings, then reduced power/speed limiting, then a severe derate (e.g., a few mph) or no-restart after key cycles ✓
• B. The engine shuts off instantly with no warning
• C. Nothing happens; it's only a suggestion
• D. The brakes are disabled

Answer: A — A stepped escalation: dashboard warnings, then reduced power/speed limiting, then a severe derate (e.g., a few mph) or no-restart after key cycles. EPA/CARB rules require an escalating inducement so operators can't simply ignore an empty or contaminated DEF tank (or SCR tampering): first warnings/chimes, then progressive power and speed reduction, and finally a severe derate — often limiting the truck to a crawling speed or preventing restart after the next key cycles/refuel. It is a deliberate, staged push to force a fix, never an instant shutdown, never optional, and it never disables the brakes.

30. A shop measures a clogged/ash-loaded DPF. What are the appropriate options?

• A. Just clear the codes and return the vehicle
• B. Run a forced regen for soot, and for ash perform off-vehicle DPF cleaning or replace the filter — clearing codes alone does not fix it ✓
• C. Remove the DPF and run an empty pipe
• D. Pour DEF directly into the DPF

Answer: B — Run a forced regen for soot, and for ash perform off-vehicle DPF cleaning or replace the filter — clearing codes alone does not fix it. A loaded DPF is addressed by burning soot with a forced/parked regen, and removing accumulated ash by off-vehicle thermal/pneumatic cleaning or by replacing the filter when it's at end of life. Simply clearing codes doesn't remove soot or ash and the problem returns. Removing or gutting the DPF is illegal tampering (and fails emissions), and DEF never goes into the DPF (it's for the SCR). Fix the cause of excess soot too, or it reloads.