New emissions certification requirements for medium duty vehicles (MDV) meeting chassis dynamometer regulations in the 8,500 lb to 14,000 lb weight classes as well as heavy duty (HD) engine dynamometer certified applications in both the under 14,000 lb and over 14,000 lb weight classes employing large diameter exhaust pipes (up to 4″) have created new exhaust stream sampling concerns. Current On-Board-Diagnostic (OBD) dyno certified particulate matter (PM) requirements were/are 7x the standard for 2010-2012 applications with a planned phase in down to 3x the standard by 2017. Chassis certified applications undergo a similar reduction down to 1.75x the standard for 2017 model year (MY) applications. Failure detection of a Diesel Particulate Filter (DPF) at these low detection limits facilitates the need for a particulate matter sensor. With the active sensing elements of the particulate matter (PM) sensors extending less than ½″ into a 4″ ID exhaust pipe, the question arises of where to locate the PM sensor to ensure it sees a properly mixed exhaust stream. Packaging and warranty requirements dictate the sensors be located near the outlet of the DPF cone, but generic fluid dynamics requirements dictate ten tube diameters after the outlet of the DPF cone. Experiments were conducted at the Ford Motor Company's Vehicle Emissions Research Laboratory on a medium duty vehicle (chassis certified application) with a diesel engine and an aftertreatment system containing a diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) catalyst, and a diesel particulate filter (DPF) utilizing both artificial and induced actual DPF faults. Several downstream DPF axial locations were selected at distances between 5 times and 18 times the diameter of the exhaust pipe from the DPF outlet. Real time PM measurements were performed at multiple sample points of each axial location (soot plane) to map out the PM distribution in the exhaust pipe. As part of this series of experiments a few DPF failures initiated during drop-to-idle (DTI) DPF regeneration were monitored with PM instrumentation. Additionally, Computational Fluid Dynamics (CFD) analyses were performed to predict mixing efficiency of the PM at each of the axial locations of the exhaust system. Both experimental and computational data will be presented.