This probably answers my question it's for a Freelander tho.
http://www.freel2.com/gallery/albums...83/FL2_DPF.pdf
NOTE:
A relatively clean DPF will return a voltage of
approximately 0.5V. This rises to approximately 4.5V
for a full DPF.
If the differential pressure sensor returns a voltage
of 4.5V or more to the ECM, and if all other
conditions are met, regeneration of the DPF will
occur.
In the event of a fault, the engine power output is
reduced by the ECM by means of a reduction in the
injected fuel quantity and EGR will be switched off