A spark plug assembly includes a spark plug, where the spark plug includes a high voltage connector, an insulator body, a metallic shell, and an electrical conductor at least partly disposed in the insulator body and the metallic shell. The spark plug assembly includes a detection unit having a transmitter device and a receiver device. The transmitter device is coupled to the spark plug and is electrically disposed between the high voltage connector and the electrical conductor. Further, the transmitter device is configured to draw an excitation current from the electrical conductor. The transmitter device includes an optical signal generator that is configured to generate an optical signal in response to the drawn excitation current. The receiver device is disposed in optical communication with the transmitter device and configured to receive the optical signal from the transmitter device.