Tisbury Motors Ltd


A lot has been written and spoken about the DPF since its entry into the UK Diesel passenger car market. The aim of this page is to try and answer some of the FAQ.

What is the DPF Filter?The diesel particulate filter (or DPF) is a device designed to remove particulate matter (PM)from the exhaust gases of a diesel engine.


The “Max” volume of PM that the vehicle is allowed to emit is set by the EU and is re-visited every 4-5 years to be reviewed. (and normally lowered)

Has My Car Got a DPF Filter?
The exhaust emissions standards for new cars have effectively required fitment of a DPF in the exhaust of diesel cars since 2009 when the 'Euro 5' standard came into force. In fact, many cars registered well before 2009 will have had one fitted too in anticipation of the change in standards. Some vehicles as early as 2003/4 will have one fitted.

How does it work?
The filter acts in a similar way to your air filter, but where as your air filter stops the dirt from entering your engine this filter stops the PM (soot to me and you) from leaving your exhaust system.

The filter is held in a metal housing which from the outside looks just the same as any silencer within the exhaust system.

The wall-flowed filter usually removes 85% or more of the soot and under certain conditions can attain soot removal efficiencies of close to 100%.

The filter is no different from any other filter in the fact that it will need cleaning from time to time.

How does the DPF system know when it’s full?

All diesel engines are controlled by a computer nowadays and vehicles with a DPF have two extra sensors fitted either side of the filter housing. These sensors measure the pressure difference between the “IN” side of the filter and the “OUT” side of the filter.

An empty filter will have a very small pressure differential with a blocked filter having a large pressure differential.

How do we Clean A DPF Filter?
Cleaning the filter is called “Regeneration” and this removes the accumulated soot from the filter.

The temperature of the DPF is important for this cleaning operation to work correctly and is therefore monitored by the vehicles engine ECU (Engine Control Unit)

There are two main ways in which the cleaning is carried out, firstly if the temperature is correct the process is constant and automatic without any intervention from the engine management system. This would normally be at higher vehicle and engine speeds. (eg. Motorway driving)

The second way is via intervention of the ECU to raise the temperature of the filter so that the regeneration can take place. This can be done in various ways, but most involve the use of extra diesel fuel injected into the combustion chamber/exhaust system to raise the temperature.

This is called “active regeneration” and is broken down into three separate stages.

Stage one; The ECU having monitored the system pressure being high (perhaps after a period of town driving) will start the active regeneration phase as soon as the driving conditions allow it to do so. The driver will not notice any change in the vehicles behaviour and no warning lights will come on the dashboard.

When for any reason the ECU cannot perform an active regeneration on that particular drive cycle (perhaps you have reached your destination and stopped the vehicle) it will try again the next time the vehicle is used.

If the driving conditions continue to be unfavorable for the ECU to carry out an active
regeneration stage two will be implemented. (Remember that the soot loading will be
continuing to rise whilst you are driving the vehicle)

Stage two; The ECU will let you know that it has tried to carry out an active regeneration and failed, it does this by turning on a warning symbol on the dashboard. Which if you read your vehicle handbook will ask you to drive the vehicle in a manner that allows the regeneration to take place.

Not running the regeneration drive cycle soon enough increases the risk of engine damage and/ or uncontrolled regeneration (thermal runaway) and possible DPF failure.

Normally the light will go out if and when the regen is complete.

When the light is ignored or multiple attempts fail, stage three will be implemented by the ECU. This involves the vehicle running in a “limp home” mode with limited power (this is because the exhaust system is partially blocked and full power would be unsafe)

Note; Fuel consumption can increase as a result of failed regenerations.

The system can only be regenerated / replaced with the use of diagnostic equipment at a suitably equipped garage.

Please feel free to contact Tisbury Motors for all your DPF issues including Removal.