Aftertreatment systems keep modern diesel trucks legal, and they cause a big share of breakdown calls.
The diesel particulate filter (DPF) traps soot and has to burn it off through regeneration. The selective catalytic reduction (SCR) system needs clean diesel exhaust fluid (DEF) and working sensors to cut NOx. When any piece of that chain fails, the engine computer steps in with warning lights and, eventually, a derate.
The harbor duty cycle is close to a worst case. Trucks crawling in terminal gate queues and stop-and-go traffic on Harry Bridges and Anaheim rarely get the long, steady highway runs that let a DPF clean itself passively. Short hops and long idles build soot fast, and the active regen the truck wants to run keeps getting interrupted by the next gate. Deleting the system is not an option — it is illegal under California Vehicle Code § 27156 and it will fail a Clean Truck Check. Fixing it is.
We come to the truck, read the aftertreatment faults and live data, and figure out whether the fix is a regen, a sensor, a DEF system repair, or something that needs a shop.
What our DPF, DEF & Regen Service covers
- Parked (stationary) and forced regens with diagnostic software, when system conditions allow
- Diagnosing why a truck won’t regen on its own — failed sensors, exhaust leaks, or regens that keep getting interrupted
- DPF differential pressure sensor and sensor line replacement
- Exhaust gas temperature (EGT) sensor replacement
- NOx sensor diagnosis and replacement, inlet and outlet
- DEF pump, DEF heater, and DEF dosing valve troubleshooting
- Clearing crystallized DEF from dosing valves and lines
- DEF quality problems — draining contaminated or degraded fluid and refilling the tank
- DEF level and quality sensor faults
- Aftertreatment fuel doser checks on engines that use one
- Exhaust leak repairs at clamps and gaskets that throw off sensor readings
- Clearing derates once the underlying problem is fixed
What can't be done on the roadside
- DPF cleaning — an ash-loaded filter has to come off and be cleaned on a DPF cleaning machine or replaced; a regen can’t burn ash
- DOC or SCR catalyst replacement, depending on access and part availability
- Root causes of heavy soot loading that are internal to the engine, like injector or turbo problems, may need shop time
- Emissions deletes or tampering — illegal under federal law, and not a repair
Fix it on site or tow it?
Most aftertreatment problems can be diagnosed, and often repaired, on site. A plugged DPF that just needs a regen, a failed NOx or temperature sensor, or crystallized DEF in a dosing valve is exactly the kind of job a mobile mechanic handles.
A shop visit makes more sense when the DPF is loaded with ash and needs to come off for cleaning, when a regen fails because of an internal engine problem, or when the truck is too derated to move to a safe work location. If the truck can still move at reduced speed, get it off the highway to a truck stop or yard where a parked regen can run safely.
What to tell dispatch when you call
The more dispatch knows up front, the better the odds the mechanic shows up with the right parts and tools on the first trip.
- Which lights are on: DPF, high exhaust system temperature, DEF, check engine, or stop engine
- Whether the DPF light is solid or flashing
- How derated the truck is — normal power, reduced power, or crawl speed only
- Any recent DEF fill, especially from an unfamiliar source
- When the last regen ran and whether a parked regen was attempted
- Engine make and model
Truck makes & engines we work on
- Freightliner
- Peterbilt
- Kenworth
- Volvo
- Mack
- International
- Western Star
Class 6 through Class 8 trucks, including engines from Cummins, Detroit Diesel, PACCAR, Volvo, Mack, Navistar, and Caterpillar.
DPF, DEF & Regen Service around the harbor & the South Bay
Regen and DEF calls cluster where trucks idle and creep: terminal gate queues on Terminal Island and at the Long Beach piers, the surface routes through Wilmington, and the warehouse districts in Carson and Rancho Dominguez. The classic one is a derate that hits on Seaside Avenue or the Vincent Thomas approach a few minutes after the truck clears a long queue — soot loaded sitting still, then faulted once it started moving, in the one place with nowhere to stop.
Freeways
Service areas
DPF, DEF & Regen Service FAQ
Can you do a forced regen on the side of the road?
Often, yes, when the truck is parked somewhere safe and the system allows it. A parked regen produces very high exhaust temperatures, so the truck needs to be away from dry grass, fuel spills, and anything that can burn. On a narrow shoulder, moving to a lot first is safer.
Why won’t my truck regen on its own?
Common causes include a failed differential pressure or exhaust temperature sensor, an exhaust leak, an EGR problem, or a duty cycle with so much idling and so many short trips that a regen never gets to finish.
What happens if I ignore a DEF warning?
Most trucks step down through escalating derates, and the final stage can limit the truck to crawl speed. Fixing the problem early is faster and cheaper.
Can bad DEF cause problems?
Yes. Contaminated or degraded DEF can trigger quality faults, crystallize in the dosing system, and damage the SCR catalyst. Store DEF out of the sun and don’t use fluid of unknown origin.
Can a regen fix a DPF full of ash?
No. Regeneration burns soot, not ash. Ash builds up over time, and eventually the filter has to be removed and cleaned or replaced.
Will you delete my DPF or DEF system?
No. Removing or disabling emissions equipment violates federal law and creates problems with inspections, resale, and warranty. We repair these systems.