Capacitor to suppress voltage spikes.
It is a stainless-steel curved notched M18X1.5 size bung and plug that you can use with wideband oxygen sensor when tune the engine.
WB D1 Wiring Harness designed to use ONLY with WB D1 AFR Controllers including D1-DIY board as supplement or D1 Wideband Controller as replacement.
WB D2 Wiring Harness designed to use ONLY with WB D2 AFR Controller Gauge.