In this article we present a design of a new miniaturized sensor for measuring exhaust particle concentrations on board the vehicles. The operation of the sensor is based on the diffusion charging and subsequent electrical detection of the particles. The sensor is characterized by ultra-fast response time, high sensitivity, and a wide dynamic range in measuring particle concentrations. The concentration response of the built prototype sensor was measured in laboratory conditions and based on the measurement results, a model for the sensor operation was developed. According to the laboratory step response measurements, the sensor signal time constant was 18 ms corresponding to a value of 40 ms for a 10 - 90% rise time. The sensor performance was also tested with actual engine exhaust in both chassis and engine dynamometer measurements. In the chassis dynamometer tests, the sensor signal recorded over a NEDC test cycle correlated very well with the total number concentration measured from the same sampling point. The fast response time was demonstrated in the engine dynamometer tests, where it was possible to adjust fuel injection of individual cylinders of the six-cylinder engine. In the tests the prototype sensor was able to detect delayed fuel injection in one cylinder. While the prototype sensor was designed to OBD applications, the design has the capacity to be used as an engine and combustion diagnostic, for example to diagnose misfiring, cylinder combustion variability, exhaust gas recirculation flowrate, etc.