A spark plug assembly includes an insulator having an upper shoulder and a lower shoulder and a metal annular outer shell having an outer surface and a cavity bounded by an inner surface. The inner surface has a flange extending inwardly to confront the lower shoulder of the insulator. The outer shell has a proximal end and an opposite fastening end. The fastening end has an external threaded region and an annular sealing seat extending outwardly adjacent the flange. The outer shell has an annular shoulder extending radially between the sealing seat and the proximal end. The proximal end has a turnover axially confronting the upper shoulder of the insulator. A sensor assembly is received about the outer shell between the shoulder of the shell and the turnover. The sensor assembly is responsive to send a signal in response to relative axial movement between the insulator and the outer shell.