Reliable means for on-board detection of particulate filter failures or malfunctions are needed to meet diagnostics (OBD) requirements. Detecting these failures, which result in tailpipe particulate matter (PM) emissions exceeding the OBD limit, over all operating conditions is challenging. Current approaches employ differential pressure sensors and downstream PM sensors, in combination with particulate filter and engine-out soot models. These conventional monitors typically operate over narrowly-defined time windows and do not provide a direct measure of the filter’s state of health. In contrast, radio frequency (RF) sensors, which transmit a wireless signal through the filter substrate provide a direct means for interrogating the condition of the filter itself. This study investigated the use of RF sensors for the continuous measurement of filter trapping efficiency, which was compared to downstream measurements with an AVL microsoot sensor, and a PM sampling probe simulating the geometry and installation configuration of a conventional PM sensor. The study included several particulate filter failure modes, both above and below the OBD threshold. The results confirmed the use of RF sensors to provide a direct and continuous measure of the particulate filter’s state of health over a range of typical in-use operating conditions, thereby significantly increasing the time window over which filter failures may be detected.