Applies to all known Euro 4/5/6 models except first generation Boxer Diesel models: MY2008/2009 Legacy/Outback as these are fitted with open-type DPF (unmanaged, no EGT sensors, no DPF differential pressure sensor, no DPF warning light, less things to worry about…).
Note that the chart represents trigger areas, not like going back and forth through all states. Y-axis is EGT at DPF inlet, the most important temperature.
- Soot accumulation < 65%: Nothing to do. Soot will accumulate or burn off passively depending on exhaust gas temperature (EGT).
Unfortunately, passive regeneration needs significant sustained engine load to be effective (reduce or maintain soot level)!
(Personal experience with Euro 4 Impreza using cruise control on motorway: actual (GPS/OBD-II) vehicle speed > 130 km/h, Euro 5+ might be better at this, though.)
- Soot ≥ 65%: If DPF is warm enough, ECU will turn on active regeneration – commence fuel post injections to actively raise DPF temperature to roughly 650°C. In case DPF being too cold, it defers when possible. Low DPF temperature has one advantage at least: less flow resistance.
- Soot > 85% and DPF temperature too low: Soot-high warning also known as vehicle speed request. Turns on DPF light steady: Ideally when there is enough engine load, EGT and DPF temperature rise to a point where active regeneration can be activated. DPF light may stay ON till soot drops below 75% (hysteresis). If soot further accumulates, state goes into red area, see next item.
- Soot > 100% – compulsory regeneration needed aka dealer visit request: One of several reasons that trigger flashing DPF light: Seems like in this state it does not do active regens anymore. Better get to dealership for diagnostics and compulsory regen as soon as possible.
- Soot > 135% – DPF Limp-Home Mode = DTC P1469 or P246C: DPF light flashing: Reduced max power. Compulsory regen not allowed anymore – without applying special tricks the DPF has to be replaced! Subaru might refuse warranty if customer had ignored flashing DPF light for too long.
ECU seems to keep this going for ~12 to 12.5 minutes, even if soot has dropped to very low percentage already, also continues when idling. Note that the soot value does not necessarily drop to 0%, depends on conditions – how quickly DPF temperature is being raised, engine load (beneficial) vs. coasting (unfavorable) etc.
If engine is turned off while active regen is in progress, it must abort the action. No harm done this way except there’s probably some soot left, next regen will occur sooner and regen counter is not being incremented. Likewise, distance since last regeneration parameter is not being reset to zero. Next time the engine is running, ECU gets to analyse DPF state and decides as usual, AFAIK not really taking any previous regen into account.
Just do not rely on the regen counter for comparisons – short-distance operation will result in very low numbers including high mileage interval values as ECU rarely gets all the time needed to complete the process.
Done in parking, needs special software (SSM-III, SSM4) that dealerships have, we can do it, too.
On Euro 4 models ECU revs the engine up for 10 minutes (does not wait till all soot is burned off, e.g. may stop at 25% soot), 15 minutes on Euro 5.
Engine speed follows a pre-defined graph, using 2.600 rpm mostly, in order to achieve some engine load due to friction.
Any active (including compulsory) regenerations cause additional engine oil dilution. See post Oil Dilution Graph.
Euro 4 vs. Euro 5
So far we’ve seen mostly same constants e.g. max soot capacity 26 g in both ROM types, looks like DPF hardware is similar. The reason why Euro 5 usually get significantly larger distances between DPF active regenerations is the result of producing less soot in the first place. This is due to smaller injector holes, higher average fuel pressure plus overall engine refinements. I would assume the downside of smaller injector holes is increased sensitivity regarding fuel quality.
Recorded Values / Statistics
Mileages with DPF light ON and DPF light flashing are being recorded. Flashing light distance > 1.000 km seems to switch into DPF limp-home mode independent of soot level.
Euro 6 adds some advanced parameters for statistics/driving style apart from recording max injection amount, max vehicle speed etc.
Temporary firmware for extended compulsory DPF regeneration
Late 2011 dealerships were provided special PAK files, flashable onto Euro 4/5 models, allowing modified compulsory regeneration at workshop. Normally in cases were soot had risen > 135% somehow, logging DTC P1469 DPF limp-home mode, the only official remedy was to replace the expensive DPF because at this level even compulsory regeneration is denied by the ECU. Basically, special ROMs allow soot levels up to 170%. Indicating such peculiar ROM, the DPF light will flash more rapidly like this: . Forced regen also takes a lot longer (45 min vs. 15 min). Whether saving the DPF has worked out (DPF light remains off) or not, this software must be replaced with normal ROM as the temporary software is not suitable for driving.
Apart from being illegal almost everywhere, stock ECU firmware does not cope too well with DPF mechanically removed. The software must be adjusted as well to avoid side effects like active regenerations and possibly limp mode later on.
Euro 4 models: ROM code enforces an active regeneration every ~6.500 km, not caring about (fake) sensor values.
Crossing water (creeks)
Since DPF sits down low and gets very hot (> 650°C, 1170°F) during active regeneration, getting in contact with water it may warp due to temperature shock and therefore be destroyed according to Australian forums.
- Automobile Association (AA) – DPFs can be problematic https://www.theaa.com/motoring_advice/fuels-and-environment/diesel-particulate-filters.html
- 2017-03 added special topics, minor refinements