I have a 2003 Kia Optima LX (DOHC 24V) - I unscrewed what appears to be a plastic electrical piece from the back of the engine, near the top. It has one black and one white wire. It looks like it broke off inside the engine. What is this piece?

the piece looked similar ( not exactly ) like the engine coolant temperature sensor, which is what I thought it was - but instead of brass, it appears to be black plastic.

Take/show this broken piece to the auto parts guy and see if it can be identified. Coolant sensors are usually brass or stainless steel but the outer half could be plastic where the wires attach. If there is a check engine light on, this will be a good clue as to what it is. Have it scanned.