THAT IS A SENSING RESISTOR THAT IS USED TO TELL THE CONTROLLER , TO SHUT DOWN IF ONE OR MORE OF THE GLOW PLUGS FAIL , IT SENSES THE VOLTAGE DROP ACROSS THE RESISTOR AND IF THE VOLTAGE GOES UP TO HI, IT WILL SHUT THE PREHEAT GLOW PLUG RELAY OFF, IF YOU USE THE CONTROLLER IT WILL HAVE TO BE THERE OR IT WILL JUST CLICK THE NO ONE RELAY AND THEN SHUT IT OFF, CIRCUIT IS ALMOST IDENTICAL TO THE C223 UCNIT EXEPT FOR THE EXTRA FUSES AND THE HEAT POSITION ON THE IGN SW, WITH OUT THAT EXTRA TERMINAL ON THE IGN SW JUST WIRE IT UP JUST LIKE THE C223 AND IT SHOULD WORK FINE JERRY