99NJMAX

yesterday I was driving to the airport and this branch flew off the median and got stuck under my car. Next thing I know I'm thorwing codes. I got 3 O2 sensor codes. I was just wondering what the difference between montior faults and voltage faults are. I'm pretty sure montiro fault means i need new sensors, but what about voltage?
Also I was checking out the o2 sensors at courtesy nissan and there are two differnt sensors for the front and back. Are there and differences in the two sensors? the rear are much cheaper than the front. And another thing I just want to be sure of. which sides of the car is the Left head?
thanks in advance