For the Smog Test:
Fuel delivery to an engine's combustion chambers are controlled by the vehicle's emissions computer or engine control unit (ECU) and fuel injectors. In order for an engine to pass a smog inspection, fuel delivery must be preciously controlled to produce the least emissions. If the injectors do not present adequate fuel to the combustion chambers, this causes low CO emissions and a lean fuel mixture causing high HC. In a situation where the injectors present too much fuel, this causes high CO emissions and a rich fuel mixture resulting in high HC emissions.
The main component responsible for letting the computer know how much fuel is in the exhaust system at any given moment, is the Oxygen Sensor. The Oxygen Sensor sends an electrical signal to the ECU, letting the computer determine exactly how much fuel it should continue to deliver to the combustion chambers. If for any reason the 02 sensor does not send accurate information to the computer or is lazy in sending the correct signal, the fuel delivery program will be altered.
Operation: During normal operation an oxygen sensors voltage should be switching between 0.1 to 1.0 volt at a rate of approximately 50 cycles per minute. An O2 sensor voltage above 0.45 volts is interpreted by the ECM as a rich exhaust, and an O2 sensor voltage signal below 0.45 volts as a lean exhaust.
The ECM's responsibility is to keep the oxygen sensor voltage switching between high and low voltage for optimum fuel efficiency, lowest emissions and highest miles per gallon. Usually the first sign of a damaged or defective oxygen sensor is poor fuel economy.
In Case of Failure: The average lifetime of an unheated oxygen sensor is 50,000 miles, and the heated oxygen sensor, 100,000 miles. Needless to say oxygen sensors require periodic replacement. Failure to do so will more then likely cause a smog check failure. 4 out of 10 high CO failed smog checks will be due to a defective, damaged or lazy oxygen sensor. It is probably one of the most underrated emissions components and should be payed much closer attention.
Often passing the smog inspection is a simple as installing a new oxygen sensor. We recommended an experienced auto smog or emissions repair mechanic perform the removal, installation and diagnosis of this component.