Diesel particulate filter will not regenerate

The filter fills with soot, the warning lamp comes on, the engine tries to burn the soot off and cannot, and eventually the engine management lamp joins in and the power goes. Owners in the cited club threads single out the earliest automatic fourth-generation diesels as the ones that give the most trouble, and report that dealer forced regenerations on those trucks can fail repeatedly. Two of their findings are worth carrying into any diagnosis. First, a blocked filter is detected by a pressure difference across it, so a fault in that sensing circuit produces the same warning as a genuinely blocked filter — one member found the filter almost clean after all the warnings said otherwise. Second, soot burns and ash does not: a filter full of ash will not respond to any amount of regeneration, forced or otherwise, and needs cleaning or replacing. The reason the triage is not simply drive-normally is what happens next when regeneration keeps failing — see the engine-oil level entry for this generation.

Gen 42007–2009UK · EUDamages other partsCommunity consensus

Drive gently — repair soonManeje con cuidado — repare pronto

Community consensus. This entry is not backed by factory documentation — treat its values and steps as a starting point, not as if they came from the factory manual.

Consenso de la comunidad. Esta entrada no se apoya en documentación de fábrica — tome los valores y los pasos como punto de partida, no como si vinieran del manual de fábrica.

Symptoms

The particulate filter warning lamp is lit, or flashing
The engine management lamp joins the filter lamp
The truck loses power, especially pulling a hill or loaded
Fuel consumption gets far worse for a stretch of driving and then recovers

Likely causes

The filter is loaded with soot that has not been burnt off
The truck only does short, cold trips, so a regeneration is started and never completed
The differential pressure sensor or its pipework is faulty, so a clear filter is reported as blocked
The filter is loaded with ash, which no regeneration can burn away

Diagnostic steps

Have the stored fault codes read and written down before anything is cleared, so you know whether the system is reporting an overloaded filter or a sensing faultPoints toThe filter is loaded with soot that has not been burnt off · The differential pressure sensor or its pipework is faulty, so a clear filter is reported as blocked
Be honest about the trips the truck actually does — short, cold, urban journeys are the single biggest cause of this problemPoints toThe truck only does short, cold trips, so a regeneration is started and never completed
Take it for a long, steady run on an open road at a constant speed and see whether the lamp goes out on its ownPoints toThe truck only does short, cold trips, so a regeneration is started and never completedRules outThe differential pressure sensor or its pipework is faulty, so a clear filter is reported as blocked
Inspect the differential pressure sensor and both of its small pipes for splits, soot blockage and corrosion before condemning the filterPoints toThe differential pressure sensor or its pipework is faulty, so a clear filter is reported as blocked
Have a workshop run a forced regeneration with diagnostic equipment and see whether the load actually fallsPoints toThe filter is loaded with ash, which no regeneration can burn awayRules outThe filter is loaded with soot that has not been burnt off

Fix paths

Give the truck a long run regularly

$Difficulty 1/5

The cheapest fix on this page, and the one that prevents the rest. A sustained run at a steady speed on an open road lets the filter burn its soot off the way it was designed to.

Repair the differential pressure sensing circuit

$–$$Difficulty 3/5

A split pipe or a soot-blocked take-off is cheap, and it makes a healthy filter look dead. Rule it out before spending anything on the filter itself.

Forced regeneration with a correct-specification oil change

$$Difficulty 3/5

One member of the cited thread got out of this with a correct-specification oil change, a clean and a diagnostic reset rather than new parts. Change the oil as part of the job: repeated failed regenerations put fuel into it.

Specialist clean, or replace the filter

$$$–$$$$Difficulty 4/5

For an ash-loaded filter, which no regeneration will clear. Removing the filter instead is illegal for road use in many jurisdictions and will fail inspection where filters are checked — the rules vary by market, so check yours before anybody offers you that as a repair.