What is the function of the oxygen sensor (O2 sensor) located upstream of the catalytic converter?

1. To supply pure oxygen gas directly to the intake manifold to increase engine horsepower
2. To measure the amount of unburned oxygen in the exhaust gases so the ECU can adjust the air-fuel mixture
3. To heat up the exhaust gas to burn off soot before it exits the tailpipe
4. To measure the oil level inside the engine crankcase