Next generation internal combustion engines and ignition systems will utilize new strategies to provide significant environmental and economic benefits, not only limited to fuel economy. Current ignition systems may use schemes including: manipulation in timing, unique fuel mixtures and injection methods, and novel physical designs as a way to more efficiently transform ignition energy. Next generation systems may also include developments of completely unique methods as compared to current spark plug systems. One such solution being developed is the Coaxial Cavity Resonator Ignition System (CCRIS), a new approach to igniting fuel air mixtures, dramatically reducing energy consumption. At the core of this ignition system is the Quarter Wave Coaxial Cavity Resonator (QWCCR), a high power microwave transformer capable of creating a low temperature plasma corona for the ignition of a variety of fuel mixtures. What is also missing from all of these next generation systems is an equally novel method for on-board diagnostics. This microwave resonator can be used as both an ignition device, because of its ability to step up voltage and form a coronal plasma, and a sensing device, because of its inherent resonance structure. During the ignition process, precise timing and energy delivered to the cylinder’s air-fuel mixture will create this desirable ignition process. Likewise, during the sensing process, the resonance characteristics of the QWCCR will change predictably during all parts of the engine cycle, and be able to detect process events such as change in temperature and pressure as well as operational events such as frequency changes, pre-ignition, complete misfire, or detonation during the monitored cycle, performing this real-time and not as an after event or downstream event. Experimental validation of this process has been obtained using a single cylinder Waukesha Cooperative Fuel Research (CFR) engine with a variable compression ratio, which will be representative of smaller gasoline engines spanning a wide range of compression ratios that are used in the transportation sector. Results indicate that a series of in-cylinder conditions can be monitored and/or measured with only a single sensor, the plasma igniter. This technology and methodology can be used to revamp on-board diagnostic systems and as a new portal for interaction with the next generation of ignition and fuel distribution systems.