[Manolito]

These are the wires between the sensor and its plug. You are splicing in a generic sensor, correct? Those wires can be any color the manufacturer of the sensor wants, and I am guessing that the , but my best guess would be: Black is signal positive and gray is signal negative, and the two whites are the heater, which should be reversible as the heater is not grounded at the sensor.

If you trace the wires to above the plug, and you tell us the model, we can tell you which wire goes where in the car harness. I just looked at an early 318 and a late 328 wiring diagram. The systems are quite different, but the colors at the plug are the same, red/white or green is 12V to the sensor heater, brown is ground to the heater. Black is signal ground and Yellow is signal positive.

If I may offer some unsolicited advice, a wise old mechanic once told me that diagnosing problems related to poor connections to an oxygen sensor can be a real PITA. Many people have done it successfully, but using generic fit oxygen sensors is probably not the place where you want to save money.