Efforts to develop a sensor for on-board diagnostics (OBD) of diesel vehicles are intensive as diesel particulate filters (DPFs) have become widespread around the world. This study presents a novel sensor that has been successfully tested for OBD diagnosis of damaged DPFs. The sensor is based on the "escaping current" technique. Based on this, a sample of exhaust gas is charged by a corona-ionized flow and is pumped by an ejector dilutor built in the sensor's construction. While the majority of ions return to the grounded sensor's body, a small quantity is lost with the charged particles exiting the sensor. This "escaping current" is a measurement of the particle concentration in the exhaust gas. Such a sensor has been developed and tested in real-exhaust of a diesel car and a diesel engine. The sensor provides high resolution (1 Hz, 0.3 s response time) and high sensitivity superseding OBD requirements. The sensor was used on an engine to monitor the efficiency of damaged DPFs. The signal was found to perform similar to the smokemeter, a widespread instrument used for routine testing in automotive laboratories. The sensor was then installed in the exhaust of a vehicle to test the DPF efficiency of a well-operating and a damaged DPF over a transient test. The sensor was found to be sensitive enough to clearly detect a defected from a well-operating particle filter even at levels as low as 6 mg km-1. This study demonstrates that a soot sensor based on the escaping current technique has the potential to be used for OBD and DPF control on forthcoming light-duty and heavy-duty diesel vehicles.