There can be multiple lambda (Oxygen) sensors along the exhaust system. I recently attended a short Bosch training course on lambda sensors and posted this text when I got back:
For the record:
The lambda sensor's main purpose is to reduce emissions. It measures the amount of oxygen in a car's exhaust gases and adjusts the fuel/air mixture to compensate. If its voltage is low, the ECU sees this and richens the mix (longer injection duration) and vica versa if voltage is high, to get the perfect mix for clean combustion.
When working ok, and on constant throttle/idle, the sensor should alternate between high and low voltage around once a second (it is a simple device with no state for 'mix is just right').
Under acceleration, the ECU ignores it and adds more fuel, so in this case the sensor would be 'high' for too much fuel.
I won't go into more detail
in the post here:
http://www.peachparts.com/shopforum/showthread.php?threadid=44233&highlight=sensor
good luck!