Where is the O2 sensor located on a 1998 GMC Sonoma?

There are two sets of oxygen sensors on late model GM trucks. There are dual catalytic converters, one on each pipe coming off of the engine. In front of the catalytic converters are the first set of oxygen sensors. These sensors check the exhaust gases to see how the engine is performing. Based on the gases, the sensors send a signal to the computer telling whether to richen or lean out the air/fuel mixture. This insures optimal performance and fuel mileage. After the converters is another set of oxygen sensors. These sensors check the exhaust gases for cleanliness after having passed through the converters. These sensors don't directly affect engine function, instead they measure how well the converters are working. If a converter is no longer working properly or is clogged, they will lean out the engine so it will continue to function for a while, causing the 'check engine' light to come on. This is probably too much information, but I just wanted to be helpful. -Neal C.-