There are many factors which will cause the failure of the DPF on the BMW 5 series but a major flaw in its design is that its mounted midway down the exhaust system. The DPF requires extremely high temperatures to burn off soot particles (temperatures can reach in excess of 700 C) and therefore the further down the exhaust the filter is mounted, the lower the temperature of the exhaust gases are before they reach the filter. This means that a regeneration (reburn) of the filter will be harder to achieve & maintain.