Upstream oxygen sensors monitor the fuel-to-air mixture in the exhaust gases of a vehicle prior to the catalytic converter and send a signal to the vehicle's onboard computer to adjust the mixture if it is too lean or too rich. The device has a platinum-coated bulb that reacts with oxygen to produce electricity that provides the signal to the computer.
The oxygen sensor must be hot to work correctly. Early generation units were dependent on the vehicle exhaust gases for heat, but later models include an electrical heating element to bring them to the required operating temperature quickly. With the older units, the lack of a signal when the engine was cold prevented the computer from making necessary adjustments. As a result, cars tended to be less efficient when the driver first started the engine.
Oxygen sensors compare the fuel mix to the oxygen in the atmosphere. At the ideal fuel-to-oxygen ratio of 14.7 to 1, the sensor produces approximately 0.45 volts of electricity. When the ratio is rich, there is little unburned oxygen and the sensor produces 0.9 volts. If it is operating with lean conditions, the electrical production drops to 0.2 volts. The computer makes continual adjustments to keep the ratio at the ideal mix.