What the DPF does
The diesel particulate filter (DPF) traps soot from the exhaust. Over time the filter fills up. To clean it, the truck burns the soot off with heat. That process is called regeneration, or "regen".
If the filter never gets cleaned, back pressure rises, the truck loses power, and the engine can derate. Knowing the four types of regen helps you keep that from happening.
1. Passive regeneration
Passive regen happens on its own while you drive. When exhaust gets hot enough, usually at steady highway speed and load, soot burns off slowly. Fleet Maintenance puts the threshold at exhaust temperatures above about 350 °C (about 660 °F).
You don't do anything, and no lamp comes on. Long highway runs make the most passive regen.
Seeing a light on the dash? Snap it with DERATE and know if you can keep driving
2. Active regeneration
When passive regen can't keep up, the engine computer starts an active regen while you drive. It adds a small amount of fuel ahead of the diesel oxidation catalyst (DOC) to raise exhaust temperature and burn soot faster.
You may see the HEST (high exhaust system temperature) lamp. That is normal. It means the exhaust is very hot. Don't park over dry grass or near anything that can burn while it's on.
3. Parked (stationary) regeneration
If active regens keep getting interrupted, for example by short trips, city work or long idling, soot builds up and the truck asks for a parked regen. A solid DPF lamp usually means a regen is needed. A flashing DPF lamp usually means you need to do it parked, soon.
How to do it (always follow your OEM manual):
- Park in a safe, open spot, away from buildings, fuel, grass and people.
- Set the parking brake and put the transmission in neutral.
- Make sure the hood is closed and nothing is touching the exhaust.
- Press the regen switch or start it from the dash menu.
- Stay with the truck. The engine will rise in rpm. Most parked regens take roughly 20 to 60 minutes, depending on the truck.
- Don't drive off or shut down until the truck shows that the regen is done.
4. Forced (service) regeneration
When soot load is too high for a parked regen, a technician runs a forced or service regen with OEM software. This is a shop job. It is also the step before the filter has to be removed for cleaning.
Soot vs ash
Regen only burns soot. Ash comes from oil additives and wear metals and does not burn. Once enough ash builds up, the filter has to come off for a professional clean or be replaced. One clue: differential pressure stays high even right after a successful regen.
Why some trucks regen too often
- Short trips, drop-and-hook or stop-and-go work
- Lots of idling
- Regens interrupted by shutting the truck off
- A bad DPF differential pressure sensor (SPN 3251) or exhaust temperature sensors
- Upstream problems: stuck EGR valve, leaking injector, turbo or boost issues
- Cold weather
If you are regenerating far more than normal, the regen is not the problem. Something is making too much soot or a sensor is lying. Fix the cause.
When to stop and call a mechanic
Call a shop if the DPF lamp is flashing and a parked regen won't start or won't finish, if the red stop lamp comes on, or if the truck is already in derate. Don't keep driving on a flashing lamp hoping it clears. Codes like SPN 3251 FMI 0 (DPF pressure too high) need a real diagnosis.
FAQ
Can I drive during an active regen?
Yes. Active regen is designed to run while you drive. Just keep going if you can, so it finishes.
What happens if I ignore the DPF lamp?
Soot keeps building. The truck will usually derate, and it may end up needing a forced regen or a filter cleaning at the shop.
Is the HEST lamp a problem?
Usually not. It means the exhaust is very hot during a regen. Keep away from flammable stuff and don't park over dry grass.