A particulate filter is a wall-flow ceramic in the exhaust. Soot collects on the channel walls and has to be burnt off. Ash does not burn off. Diesel cars meet this limit constantly, because a diesel spray makes soot and many of those cars never hold the exhaust hot for long enough. Direct-injection petrol engines make fewer particles but enough that later ones carry a filter of the same general type. The chemistry, the difference between passive and active regeneration, and the legal position of a missing filter are on petrol and diesel emissions. This page is what goes wrong, and what a warning lamp does and does not prove.
Removing the filter, or leaving the shell in place and emptying it, is not a repair described here. The AA records that using a vehicle modified so that it no longer meets the emissions standard it was built to is an offence under the Road Vehicles (Construction and Use) Regulations, and that a missing filter, where one was fitted, fails an MOT. The current inspection text is the MOT manual, section 8, which is also cited on the emissions page.
Common diesel engine problems and common petrol engine problems both link here.
Soot and ash
The AA’s account of a diesel filter is the one that matches the calls its patrols see. The soot is burnt during regeneration and leaves a small ash residue. The ash stays in the filter unless the unit is removed and cleaned by a specialist. The AA’s statement of life, for a filter in a car that is used in the way the system expects, is well over 100,000 miles. That is a usage claim, not a guarantee, and it collapses when regeneration never finishes or when the engine is making more soot or more oil ash than the filter was sized for.
Regeneration therefore answers a soot load. It does not answer a filter full of oil ash, and it does not answer a pressure sensor that is lying. A workshop regeneration commanded on the wrong one of those three wastes the procedure or overheats the ceramic.
Diesel regeneration
Passive regeneration is the exhaust being hot enough, on a motorway or a fast A-road, for the trapped soot to oxidise. The AA notes that many cars are not driven that way, so the software commands an active regeneration: extra fuel is injected to raise the filter temperature. In the AA’s description this is started on the order of every 300 miles, depending on use, and takes about 5 to 10 minutes. While it is happening the driver may see the cooling fans running, a higher idle, stop-start inhibited, higher fuel use, a hot acrid smell, and a change in engine note. Those signs are a regeneration, not by themselves a failure.
The failure is a regeneration that is not allowed to finish. The AA’s list of what prevents a normal regeneration is specific:
- Repeated short journeys in which the engine never gets hot.
- The wrong engine oil. Low-SAPS oil is specified because oil additives become ash in the filter. The handbook grade is the one that applies.
- A fuel-system or EGR fault making excess soot.
- A warning lamp or a stored fault that inhibits the routine.
- An overdue service.
- On engines that meter a fuel-borne additive such as Eolys, an empty additive tank. The AA describes the additive as lowering the soot ignition temperature, stored in its own tank, and lasting on the order of 70,000 miles when full. Ignoring that lamp blocks the filter.
- A low fuel level. The AA’s general figure is under a quarter of a tank, below which active regeneration is inhibited. The vehicle strategy may use a different threshold.
A warning that the filter is loaded can often still be cleared by a drive that lets the active cycle finish. The AA’s practical description is about 10 minutes at speeds over 40 mph. Leave it, and continue in stop-start traffic, and the software goes to a restricted performance mode so the filter is not damaged further. At that point a road drive is no longer enough. A dealer or a workshop has to command a regeneration, and in the extreme the filter is replaced. The AA’s cost remark, at least £1,000 plus labour for a replacement, is a 2017 figure and only a scale.
There is only a short gap, in the AA’s experience, between a filter that is partly loaded and one that needs the workshop procedure. A differential-pressure sensor fault can turn the engine lamp on as well as the filter lamp. The sensor and its hoses are part of the diagnosis. A hose full of water or soot reports a pressure the ceramic does not have.
Oil level rising
Active regeneration on engines that use a late in-cylinder injection puts fuel into the exhaust on purpose. The emissions page separates that late injection from the close post-injection used to cut soot in the cylinder, and from an injector mounted in the exhaust, which never sends the fuel through the combustion chamber. When the in-cylinder event is cut short because the journey ends, some of that fuel remains and can pass the rings into the oil. The dipstick clue is an oil level that rises. Diluted oil loses viscosity. The turbo and the bearings then become the expensive consequence of a filter problem that was left to repeat.
This path is not universal. An exhaust injector, or a strategy that does not post-inject into the cylinder, does not fill the sump the same way. Confirm the layout before treating a high oil level as proof of failed regenerations. Fuel in the oil also has other sources, including an injector that is leaking into the cylinder.
What to check before a forced regeneration
A commanded regeneration is the right tool when the filter still contains soot, the sensors agree, and the engine can reach the temperature. It is the wrong tool when the loading is ash, when a temperature sensor is implausible, or when EGR, boost or injectors are still producing the soot. Workshop diagnostic guides converge on the same preconditions: fuel level, coolant temperature and a thermostat that actually lets the engine warm, both pressure hoses, exhaust temperatures before and after the filter, oil level and condition, and faults in EGR, air, boost and fuel. A thermostat stuck open keeps the engine cool and is one reason a regeneration never starts. Driving pattern explains a soot load. It does not explain a pressure reading that is impossible at idle, or a temperature sensor that does not rise from cold.
Calculated soot mass is a model. Differential pressure is a measurement, and it still has to be read against flow, because the same soot mass at a higher exhaust flow makes a higher pressure difference. The two disagreeing is a reason to inspect the sensor, not a reason to reset the model and hope.
Petrol filters
A petrol particulate filter, GPF or OPF, exists because direct injection makes particles that port injection largely does not. The emissions page records the AECC account: a close-coupled filter on a petrol engine usually regenerates passively because the exhaust is hot, and an underfloor filter is more likely to need help in town and stop-start use. The diesel story, a car that cannot finish an active cycle on the school run, is not the default petrol story.
The faults that do transfer are sensor faults, ash from the wrong oil or from an engine burning oil, and a misfire or a rich fault that loads the filter with unburnt fuel and can overheat it. A petrol filter warning still wants the same discipline: pressures and temperatures first, then the reason the soot or the ash is there. Inlet-valve carbon on these engines is a separate deposit, on the other side of the valve, described on carbon deposits and cleaning.
Sources
- The AA, diesel particulate filters, updated 7 August 2017. Passive and active regeneration, the signs during an active cycle, the blockers, additive tanks, restricted performance, and the Construction and Use point. Distances, times and the replacement cost are the AA’s figures.
- DVSA, MOT inspection manual, section 8. The in-service inspection of a diesel particulate filter and of other emissions equipment.
- Petrol and diesel emissions on this wiki. Wall-flow filters, soot against ash, late post-injection, and petrol filters.
- Diagnostic Central, DPF regeneration diagnostic guide. Preconditions a workshop checks before a service regeneration, including oil dilution and a thermostat that leaves the engine cool. Used as workshop practice, not as a manufacturer limit.