GMC Repair: Oxygen senors, air fuel mixture, wire sensors

Question
I have a 1999 GMC Yukon SLT with a 5.7 Liter motor I am told it depends on my Transmission as to the sensors.  Either 2 Up stream and 2 down stream.  It is single exhaust so why do I need two down stream sensors.

Answer
Hi
There are 2 sensors on each "bank"  of cylinders (left = Bank 1, Right = bank 2). There is also 2 seperate catalytic converters for each bank. 2 sensors are before (aka PRE) converter, and 2 after (POSt) converter. The 2 PRE converter oxygen sensorsare used for making air fuel mixture changes as determined by the amount of oxygen present in the exhaust stream. Watching the voltage (measured in Milli volts) these sensors vary voltage very fast or rapid, and that varing voltage is used by the PCM (Powertrain Control Module) to adjust the length of the pulse time for injectors (either ON longer or shorter). This adjusts the air fuel mixture for maxium power and efficiency. The 2 POST converter sensors vary voltage very slowly. They are used to "fine tune" air fuel mixture AS WELL AS to determine the state of the Catalyst System Efficiency. All 4 sensors are also fed 12 volts through a heater circuit to heat them quickly (more quickly than the old single wire sensors). Codes for sensors can be for a failure in the heater ciruiuts as well as slow or dead sensors OR related wiring.