What is the function of the oxygen sensor (lambda sensor) placed in the exhaust manifold of a petrol engine equipped with Multi-Point Fuel Injection (MPFI)?

1. To measure the amount of particulate matter in the exhaust to regulate the EGR valve.
2. To detect the concentration of unburned fuel in the exhaust and trigger spark plugs.
3. To measure the concentration of remaining oxygen in the exhaust to allow the ECU to regulate the stoichiometric air-fuel ratio.
4. To preheat the catalytic converter during cold start cycles.