EmiSense Technologies, LLC (www.emisense.com) is commercializing its electronic particulate matter (PM) sensor that is based on technology developed at the University of Texas at Austin (UT). To demonstrate the capability of this sensor for real-time PM measurements and on board diagnostics (OBD) for failure detection of diesel particle filters (DPF), independent measurements were performed to characterize the engine PM emissions and to compare with the PM sensor response.Computational fluid dynamics (CFD) modeling was performed to characterize the hydrodynamics of the sensor's housing and to develop an improved PM sensor housing with reproducible hydrodynamics and an internal baffle to minimize orientation effects. PM sensors with the improved housing were evaluated in the truck exhaust of a heavy duty (HD) diesel engine tested on-road and on a chassis dynamometer at the University of California, Riverside (UCR) using their Mobile Emissions Laboratory (MEL). This consisted of a HD diesel tractor/trailer unit with PEMS for characterizing exhaust. Time-resolved PM mass and/or number concentrations were measured with an AVL MSS-483, Dekati DMM-230, TSI Dustrak, and UCR's fast scanning mobility particle sizer (fSMPS) and compared to the outputs of 12 EmiSense PMTrac sensors.In situ PM measurements from EmiSense's PM sensors correlated well with gravimetric measurements, and results from the real-time laboratory PM monitoring instruments in UCR's mobile emissions laboratory. In addition, particle size distribution data from UCR's fSMPS are presented and discussed. One characteristic of the PM sensor that is desirable for post-DPF OBD measurements is that it directly measures PM in the exhaust with no additional requirements for sampling and/or dilution systems.