before and after DPF


Since the Introduction of ‘Euro 5’ standards in 2009, emissions requirements for new diesel vehicles require Particulate Filters to be fitted to the exhaust from the factory. Even before this change of regulations, many vehicle manufacturers where already fitting DPFs in anticipation of the new regulations. The ‘Euro 5’ and future standards aim to deliver reductions in soot output (diesel particulates) of up to 80%! Sounds brilliant, but these filtration systems are not free from issues causing DPF warning lights, engine limp home mode, reduced efficiency/mpg and poor engine performance.

Regeneration of the DPF is required for efficient operation of the filter and engine, this can happen passively (while driving under normal conditions) or actively (while driving or also a “forced” regeneration). The filter will passively regenerate when under normal driving conditions that allow it to reach a high enough temperature to burn off the deposits in the filter, this could be for example on the motorway. An active regeneration process is completed by the ECU causing an increased temperature of the exhaust gases passing through the filter, which then allows burning away the carbon deposits. To control this process a sensor transmits data to the engine control unit that then calculates the increase in post-injection fuel quantity required to increase the temperature enough to complete a regeneration.

As with all filters, the DPF must be regularly emptied, if regeneration is not carried out or fails the DPF will start to fill up, eventually becoming partially blocked. A lot of DPF issues begin as passive regeneration can only occur at high speeds on a motorway, or under increased engine load, so when a vehicle is continually used for shorter local journeys, where the engine cannot get up to temperature properly this can quickly lead to the DPF blocking up. 


Passive regeneration:

When the engine is under a high enough load, for example on a motorway or A Road, this will naturally cause the exhaust gas temperatures to rise, and therefore the DPFs internal temperature will hopefully reach a high enough level to start burning off filter deposits.

Active regeneration:

If the vehicle is mostly used for short journeys, or doesn’t regularly get driven on a motorway, it may mean that the DPF is never being subjected to high enough temperatures to regenerate naturally. This means soot particles will not be burned off so will start to accumulate inside the filter. This leads to a high level of soot content in the filter, once this level reaches a certain threshold, the engine’s control unit (ECU) triggers an active regeneration to be carried out at the next available opportunity. The amount of soot accumulated inside the DPF is calculated by the ECU using readings from sensors– the signals from sensors regarding flow, temperature and pressure then allows the ECU to calculate the degree of DPF saturation.

The differential pressure sensor measures the difference in exhaust gas pressure both before and after the DPF filter. As the filter becomes more blocked this pressure value will rise, once this pressure reaches the manufacturer set limit, the ECU will attempt to raise the temperature of exhaust gas to a target of around 600°, which is a temperature high enough to regenerate the filter. The ECU tells the engine to do this using a number of methods:

  • Regulating air flow through the Electronic Throttle Control (ETC).
  • Opening the Exhaust Gas Regeneration (EGR) valve so that it recycles exhaust gases through the intake increasing engine combustion temperature.
  • Adding additional fuel (post-injection) following the principal injection.
  • A second post-injection which leads to unburnt fuel evaporating inside the combustion chamber. During this stage temperatures inside the DPF can reach 620°C.
DPF Cleaning Equipment
SouthWest DPF Cleaning logo on black

Forced regeneration:

Unfortunately, it’s not always possible to allow the correct driving condition for a long enough period of time for an active regeneration to be successful. When the values recorded by the DPF for soot saturation levels reach a certain point, or when the active regeneration has failed to complete a number of times, the ECU will no longer deem it possible to complete an active regeneration of it’s own accord. This is the point when you are likely to see warning light and/or messages telling you there is a problem with your DPF!

At this point it may be up to South west DPF Cleaning to attempt to instruct your ECU to carry out a forced regeneration, depending on the vehicle this can either done while the vehicle is stationary and left to carry out the process, or while on a set driving route to allow completion of the regeneration. This process can only be done using specialised diagnostic tools and should not be attempted anywhere other than a garage with the correct safety equipment.

However in some extreme cases, even forced regenerations will fail or only allow trouble free driving for a short period of time. Primarily this will be due to continued shorter journeys, or it could be because of another engine fault that is actually causing premature DPF blocking. For example failing injectors causing incorrect/incomplete combustion, or low quality fuels.

If a forced regeneration fails its often the case that a mechanic will tell you that a new DPF is the only fix and new diesel particulate filters are not cheap! In most instances this not 100% accurate advice, even when coming from a main dealer, there may be other more cost effective options.

This diagnosis is often given when the mechanic has a limited understanding of the very complicated DPF system and therefore assumes the filter has completely failed. The good news is we very rarely see a completely failed DPF!

For many people replacing the DPF at huge expense will also only resolve the problem in the short-term, if there is a fault still present with other engine components, then it’s only a matter of time before the new expensive DPF also becomes saturated, and you are back to square one.

South West DPF Cleaning recommend an engine carbon clean every 10,000-12,000 miles in order to reduce the risk of DPF issues. We also offer a full professional DPF cleaning service, either with the filter on the vehicle or removed.