O2 sensors measure the amount of oxygen in the exhaust stream. They do this by comparing the amount of O2 there is in the ambient surrounding air vs the exhaust stream inside the manifold. The difference of O2 creates a voltage, which is sent to the car computer to tell how much O2 is in the exhaust.
A lean exhaust will result in low voltage (about 0.1 to 0.2 volts) because there is still residual oxygen molecules in the exhaust which did not react with any gasoline molecules during combustion. A rich exhaust will result in higher voltage (about 0.8 to 0.9 volts) because all oxygen molecules reacted with the gasoline. There is no free O2 in the exhaust. The car computer strategy is to fluctuate the exhaust between rich and lean, consequently shifting the voltage between high and low. An efficient O2 sensor will read this fluctuation a few times a second and report back to the car computer.
The O2 sensors upstream from the catalytic converter are supposed to fluctuate high/low rapidly. This is normal and required. The O2 sensors downstream of the catalytic converter are supposed to be quick reacting to differences in O2 levels, but the preferred voltage is supposed to stay even / level. A steady O2 voltage after the CAT means that the CAT is still efficient. A wildly fluctuating O2 after the CAT means your CAT is burned out. Theory of how Catalytic converters go beyond the realm of this explanation. Would take a lot of text to explain. So I'll stop here.
Tuesday, February 10th, 2009 AT 8:16 PM