A DPF warning light means the diesel particulate filter is loading up faster than the truck can clean it. Sometimes the fix is simply letting the truck complete a regeneration. But a filter that keeps loading, or a truck that needs parked regens more and more often, usually has a cause somewhere else: ash build-up, the truck's duty cycle, or an engine problem upstream, often in the EGR system. Finding that cause is what keeps the light off.
A quick refresher, and what this post adds
Our post Understanding the Diesel Aftertreatment System: DPF, EGR & SCR explains how the three systems work and the difference between passive and active regeneration. This post is the next step: what to do when the DPF or EGR starts causing trouble on a fleet truck.
The short version of why every modern truck has these parts: EPA's emission standards for heavy-duty highway engines set strict limits on particulate matter and nitrogen oxides. The DPF traps soot to meet the particulate limit. The EGR system sends a measured amount of cooled exhaust back into the intake to lower combustion temperature and reduce nitrogen oxides.
Soot and ash are not the same problem
This is the single most useful thing for a fleet manager to understand about DPFs.
Soot is unburned carbon. It burns off during regeneration, either passively from exhaust heat on the highway or actively when the engine adds heat.
Ash does not burn. A Cummins technical support director told Overdrive that ash in the DPF comes mainly from the additives in engine oil, and that it does not burn even at temperatures that burn off soot, so it has to be cleaned out (Overdrive). DieselNet's technical guide on DPF ash names engine lube oil additives as the main source, along with fuel and engine wear, and estimates that on a typical heavy-duty vehicle about half the material in the filter is ash after roughly 33,000 miles.
That explains a pattern we see: a truck that regenerates more and more often, and loses a little power each time. More regens do not fix it, because the space they would clear is being taken up by ash. That truck needs the filter cleaned.
What the warning lights are telling you
Dash symbols vary by engine maker, so check the operator's manual for the exact meaning on your trucks. In general:
- DPF or soot-level light, solid. The filter is loading. On many trucks this means drive at highway speed or run a parked regeneration.
- DPF light flashing, or paired with the check engine light. Soot load is high. The truck may soon reduce power to protect the engine.
- High exhaust temperature light. A regen is running. This is normal; do not shut the truck off in the middle of it unless it is unsafe to continue.
- Engine derate or stop engine light. Get the truck to a safe stop and call the shop.
Our shops treat a yellow DPF light as an early warning. Ignore it long enough and the engine may start acting up.
Why some trucks clog more than others
Duty cycle matters. Overdrive's reporting notes that a DPF burns off soot about as fast as it collects on the highway, but soot builds up during idling and slow traffic (Overdrive). Local delivery routes, job-site trucks and units that idle for long periods do not get hot enough, long enough, to clean themselves. Those trucks need more regens and, over time, more frequent cleaning than a regional tractor running interstate miles.
Interrupted regens make it worse. If drivers routinely shut down during a parked regen or postpone the request, soot keeps stacking up.
Upstream problems that load the filter
When a DPF clogs faster than its duty cycle explains, our technicians look upstream before cleaning or replacing it. Common causes:
- A cracked or leaking EGR cooler. Thermal cycling and constant system pressure can crack the cooler internally. Coolant then enters the exhaust. Signs include unexplained coolant loss, white exhaust smoke and a sweet smell. Coolant residue can damage the DPF.
- A soot-plugged EGR cooler or sticking EGR valve. This restricts exhaust gas flow, strains the engine and can increase soot output.
- Injector problems. A leaking or poorly spraying injector produces more soot.
- Oil getting into the exhaust. A worn turbo seal or other oil consumption adds both soot and ash.
- The wrong engine oil. Use the oil category the engine maker specifies. Diesel oils are licensed by service category under the American Petroleum Institute's engine oil licensing program; CK-4 and FA-4 are the current on-highway categories, and FA-4 only goes in engines whose maker approves it.
- Sensor faults. A bad differential pressure sensor can report a full filter that is not full, or miss one that is.
Cleaning a DPF without fixing these sends it back in.
When to book a check
We recommend having the DPF system checked every 300,000 miles, and sooner on idle-heavy or stop-and-go trucks, or any truck that has started regenerating more often. Book it while the truck is still running. A planned visit costs far less downtime than a tow after a derate. Keeping the EGR and DPF clean can improve fuel mileage and usable power and extend engine and component life.
When you call, tell us how often the truck has been regenerating, whether it has lost power, whether it is using coolant, and what the truck does most days. That narrows the diagnosis before the truck rolls in.
Talk to an aftertreatment tech
Statesville: 704-960-8951. Bessemer City (serving Gastonia and Gaston County): 980-391-4100. Monroe: 980-396-6750. Emergency service is available at all three shops, Monday to Friday, 7:00 AM to 5:00 PM, on each shop's own number. You can also email marketing@gallaherfleetsolutions.com or request service at www.gallaherfleetsolutions.com/contact, and we will get back to you the same business day. See EGR and DPF cleaning for the full service.