Diagnostic systems and methods for detecting leaks in an exhaust gas recirculation (EGR) system of an engine of a vehicle utilize an upstream oxygen (O2) sensor disposed in an exhaust system of the engine upstream from an EGR port of the EGR system and configured to measure an upstream O2 concentration of exhaust gas produced by the engine, a downstream O2 sensor disposed in the exhaust system downstream from the EGR port and configured to measure a downstream O2 concentration of the exhaust gas, and a controller configured to receive the measured upstream and downstream O2 concentrations from the upstream and downstream O2 sensors, respectively, and detect a leak in the EGR system when a difference between the measured downstream and upstream O2 concentrations exceeds a diagnostic threshold.