There are currently no fault codes indicating problems with any of the O2 sensors. If I am able to get the misfire problem taken care of, and the codes stay cleared out, and I do not have any codes showing for a specific O2 sensor should they all be replaced or how would I determine which one if my gas mileage does not improve? If I remember correctly there are four all together. 2 before the cats and 2 after?

 

Most DII's only have 2 O2's, both posy cat.
You wont get any O2 codes until they fail completely.
Reduced O2 performance will reduce your MPG.
I would replace the wires, clean the codes and then check your MPG over the next tank or two and see what happens.

 

Thank you for the advice. I will most certainly follow it if the wires and/or coil pack does not fix the problem. I will be downloading the Rave manual once I get home from work so that I can look up the information on changing out the O2 sensors if need be. I hope to also compile a list of different sensors and parts that I can order to have on hand so that when I need to replace something I don't have to wait for a part to come in.

 

I just did some research, both via the D2 Workshop manual and also physically looking. According to the D2 workshop manual, ALL NAS Disco II vehicles have 4 o2 sensors. 2 pre-cat and 2 post-cat. (2 front o2 sensors shown on page 442 of D2 workshop manual pdf) I have located the ones in the engine compartment, and it's too cold outside (and too much snow) to check for the other two, though I am sure they are there where they should be. As I do not have any fault codes and I have been told that they won't show codes until complete failure, which set are more likely to be causing the bad gas mileage? Originally before all problems were present I was getting roughly 14-17 mpg, now I am getting 7 mpg.

I will be replacing the plugs again next week, also finally replacing the plug wires. Hopefully this will take care of cylinder 2 and 3 misfiring. Hopefully it will also take care of the mileage problem. Other options if the misfire is still there, I will be replacing the coil and if I must, replace some o2 sensors if the mileage problem is still there, though I am not sure which 02 sensors I must start with since there are no codes telling me which ones out of the four are bad, if they're bad that is.