When monitoring a zirconia oxygen sensor on an oscilloscope under closed-loop operation at 2000 RPM, what indicates a healthy sensor?

1. A constant, flat-line output voltage fixed at exactly 0.50 volts
2. A voltage waveform oscillating rapidly between 0.1 volts and 0.9 volts, completing several cycles per second
3. A high-frequency square wave switching between 0 volts and 12 volts
4. A voltage reading that steadily climbs from 1.0 volt to 5.0 volts as the engine warms up