Toyota Repair: 1998 toyota landcruiser O2 sensor, o2 sensors, toyota landcruiser

Question
First of all thank you for this website, I have a vehicle that causes a twinkle in the eyes of the service writers.  I found out so much information here I am giddy.  My question is: I have 108,000 miles on my Landcruiser. It failed the Echeck because of the check engine light and the diagnosis is the O2sensor. how many O2 sensors are on this vehicle?  Where are they? Should I replace them all?  

Answer
There is a total of four O2 sensors, one in front and one in the rear of each cat converter, there are four catalityc converters, two for each bank, I can't recommend which ones to replace because I don't know what the trouble codes are, most of the time the Bank one or Bank two number one sensor fails, these are the two in front of the cat, if you can tell me what the DTC is that they are basing replacement of the O2 sensor is, I may be able to help more.