All info for 2009/2010 Impreza Diesel Euro 4 model unless noted otherwise.
OBD Socket, Body Integrated Unit (BIU)
Mass air flow & inlet air temperature sensor
Similar to this “Air Flow Meter” DENSO product image:
Variable geometry turbocharger control valve, common rail pump suction control valve
Manifold air pressure & manifold air temperature sensor; fuel temperature sensor
Diesel Particulate Filter (DPF)
Glossary: DPF (Diesel Particulate Filter)
DPF casing contains oxidation catalyst (left portion, roughly 1/3 length) and actual DPF.
Exhaust Gas Temperature (EGT) Sensors
Note: Only 2008/2009 Legacy/Outback models are fitted with open-type (unmanaged) DPF. These are easy to recognize as they do not have any EGT sensors nor DPF pressure difference sensor.
DPF Pressure Difference Sensor
Larger diameter piping is connected to DPF inlet (downstream catalyst), smaller diameter one to DPF outlet.
Note: On Euro 5/6 models, DPF pressure difference sensor is mounted on engine, below vacuum pump (left bottom in picture above).
Further parts: DENSO – Diesel Engine Management System