Truck regen: passive, active and parked regeneration explained

Every modern diesel truck with a diesel particulate filter has to clean that filter regularly. When regens work, drivers barely notice them. When they do not, the truck can derate, the dash lights up, and a shop visit is next. Knowing how regens work, and what the warning lamps mean, keeps most trucks out of trouble.

The aftertreatment system

Tap each part to see what it does in the regen process.

Three kinds of regen

Passive regen. At highway speeds under load, exhaust temperatures can get hot enough to burn soot continuously. The driver does nothing, and there is no lamp. Trucks that run long highway miles do much of their cleaning this way.

Active regen. When the soot load builds up and passive regens are not enough, the engine raises exhaust temperature, often by adding fuel to the exhaust stream, while driving. A high exhaust temperature lamp may come on. Keep driving normally.

Parked or stationary regen. If active regens cannot finish, for example on a truck that runs mostly short trips, the engine asks the driver to start a regen while parked. The driver parks in a safe place and starts the regen with a switch, following the engine maker's procedure. The engine runs at a raised idle until the filter is clean.

Dash lamps

Engine makers use similar symbols, though the exact meaning and stages differ:

• DPF lamp (often a box with dots): the filter is loading and needs a regen. Drive at highway speed if possible, or start a parked regen if instructed.
• High exhaust system temperature lamp: a regen is in progress and the exhaust is very hot. Keep clear of flammable materials.
• Check engine or stop engine lamp together with the DPF lamp: the soot load is high and the engine may derate; a parked regen or service is needed.

Read your engine maker's description of each lamp and stage. Ignoring early lamps usually leads to a derate and a forced service regen at a shop.

Why short trips cause problems

Passive regens need sustained exhaust heat, which comes from highway driving under load. Short trips, frequent stops and long idling keep the exhaust cool, so soot builds up faster than it burns. Day cabs and local trucks often see more active and parked regens. Our guide to the day cab truck covers local duty cycles.

Ways to help:

• Mix in some highway driving when the route allows.
• Avoid unnecessary idling.
• Do parked regens when the truck asks, rather than putting them off.
• Keep the engine in good health so it makes less soot.

When regens fail

Regens can fail or happen too often because of problems elsewhere:

• Engine problems that make extra soot: leaking injectors, EGR faults, air intake leaks, a failing turbo.
• Sensor faults that misread soot load or temperature.
• A clogged doser that cannot add fuel for active regens.
• Ash buildup in a DPF that is overdue for cleaning.
• Low voltage, since regens depend on electrical systems; see our truck battery guide.

Doing a parked regen safely

1. Park on a hard surface, away from dry grass, buildings, fuel and other vehicles.
2. Set the parking brake and follow the engine maker's procedure for starting the regen.
3. Stay with the truck, and keep people away from the exhaust outlet.
4. Do not interrupt the regen unless necessary.
5. If the regen will not start or complete, have the truck diagnosed.

DPF cleaning

Regens burn soot but leave ash, which builds up over time. DPFs need periodic cleaning or replacement on the engine maker's schedule, usually by removing the filter and cleaning it with specialized equipment. Our truck maintenance guide covers aftertreatment service in a PM program, and the preventive maintenance schedule generator can track the interval.

Emissions equipment and the law

The DPF and the rest of the aftertreatment system are required emissions equipment. Removing them, disabling regens or reprogramming the engine to bypass them is illegal under federal law, voids warranties and causes problems at resale and inspections. If regens keep failing, the answer is diagnosis and repair; our semi truck repair guide covers choosing a shop.

How often regens happen

How often a truck regenerates depends on how much soot the engine makes and how often the exhaust gets hot enough for passive regens. A highway truck may rarely need an active regen, because passive regens keep the filter clean. A local truck with lots of idling may need active regens every day or two, and occasional parked regens. A sudden change in regen frequency, such as a truck that used to regen weekly now asking daily, is a sign something has changed and should be checked.

What makes an engine produce more soot

Soot comes from incomplete combustion. Anything that upsets combustion can raise soot output:

• Fuel injectors that leak, stick or spray poorly.
• EGR system faults, such as a stuck valve or a clogged cooler.
• Air intake problems, including a restricted air filter or boost leaks.
• Turbocharger problems that reduce air supply.
• Excessive idling and very light loads.
• Low-quality fuel or the wrong oil, in some cases.

Fixing the cause often fixes the regen problem; cleaning or replacing the DPF without fixing the cause just resets the clock.

Ash vs soot

Soot burns off during regens. Ash does not. Ash comes mostly from the additives in engine oil that burn in small amounts during normal operation. It builds up slowly in the DPF and eventually restricts flow, so the filter must be removed and cleaned, or replaced, at intervals the engine maker sets. Using the oil specification the maker requires, which limits ash-forming additives, helps the DPF last longer between cleanings.

Forced regens at a shop

When the soot load gets too high for a parked regen, or the engine has derated, a shop can run a forced or service regen using diagnostic software. If the filter is too loaded, it may need to come off for cleaning instead, because a regen on a very full filter can create extreme temperatures. That is one reason to respond to early DPF lamps rather than waiting.

Regens and fuel use

Active and parked regens use extra fuel to heat the exhaust. Trucks that regen often use noticeably more fuel than those that clean passively. That is another reason to reduce idling and keep the engine healthy: fewer regens mean less fuel burned for cleaning.

Regen tips for drivers

• Learn what each lamp means on your truck, and what stage each represents.
• When the DPF lamp comes on, a stretch of highway driving may let an active regen finish.
• If the truck asks for a parked regen, do it at the next safe opportunity.
• Do not shut the engine off in the middle of a regen unless you must.
• Park away from flammable materials, especially during or right after a regen.
• Tell your shop if regen frequency changes.

Regens on box trucks and medium duty engines

Medium duty diesel engines in box trucks also use DPFs and regenerate. Because many box trucks work in city traffic with frequent stops, they can be especially prone to frequent active and parked regens. Some routes simply do not get the truck hot enough. Planning a highway segment into the week, where possible, can help the DPF clean itself.

Regens and battery health

Parked regens run the engine at elevated idle for some time, and regen controls depend on sensors and electronics. Weak batteries or poor connections can cause electronic faults that interrupt regens. Keep the electrical system healthy as part of aftertreatment care.

What it costs when aftertreatment fails

Aftertreatment repairs can be expensive: DPF cleaning or replacement, sensors, dosers and the DOC. Downtime adds to the bill. Staying on top of regens, PM service and engine health is far cheaper than a major aftertreatment repair.

A regen checklist for a new truck or driver

When starting with a new truck, or training a new driver, go through these points:

1. Read the engine maker's description of the aftertreatment lamps and stages for this engine.
2. Find the parked regen switch and the regen inhibit switch, and learn how each works.
3. Learn where the truck shows soot load, if it does, on the dash or through a display.
4. Note the DPF cleaning interval and when it was last done.
5. Agree on what the driver should do when each lamp appears.
6. Keep a simple log of parked regens and dates, to spot changes in frequency.

The regen inhibit switch

Many trucks have a regen inhibit switch that temporarily prevents active regens, for example when parked in a place where high exhaust temperatures would be unsafe, such as near flammable materials or in a building. Using it occasionally is fine, but leaving it on prevents the filter from cleaning and leads to more lamps, derates and service regens. Turn it off as soon as it is safe to do so.

What a technician checks for regen problems

When a truck has regen problems, a technician will usually check:

• Fault codes and freeze-frame data.
• Soot load and differential pressure across the DPF.
• Exhaust temperature sensor readings.
• The doser or hydrocarbon injector for clogging.
• The DOC face for plugging.
• Engine-side causes of soot, including injectors, EGR and boost.
• The DPF's ash load and cleaning history.

Fixing regen problems often means fixing something upstream of the DPF.

Regens in cold weather

In cold weather, exhaust temperatures are lower, especially at idle and light load, and passive regens are harder to achieve. Trucks may need more active and parked regens in winter. Cutting idle time, plugging in an engine block heater and keeping the truck working under load help. Be especially careful with parked regens near snowbanks and buildings, where hot exhaust can cause damage.

Keeping a regen log

A simple log helps spot problems early. Each time the truck asks for a parked regen, or a DPF lamp appears, note the date, mileage, engine hours and what the truck had been doing, such as city work, idling or highway driving. Over a few weeks, the log shows whether regens are becoming more frequent and under what conditions. Share it with your technician at the next service; it is often more useful than a single fault code.

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