95stang5.0

I am getting ready to replace my 02 sensors. Do i need 2 up stream or 2 down stream. THe people at adavanced auto told me i need 4... well let me know.

Thanks

WizardBlack

What are you replacing them for? Do you have a CEL? Do you have drivability or fuel economy issues?
If you are just doing preventative maintenance, all four should go, I suppose. They all can fail.
Here's the skinny, though.
The two front sensors (upstream) are used for fuel mixture control. They are what the car actually runs from for closed loop fuel tuning. If they go bad, you get a CEL, low power and poor economy.
The rear two sensors (downstream) are used for catalytic converter efficiency checks. They are used to make sure your cats are up to snuff. If they go bad you get a CEL. They could be dragging on the road and it wouldn't make any difference as far as power is concerned.
If you don't care about a CEL, the fronts are the only ones I would worry about to maintain performance.
If you have a CEL and are trying to fix it, have Autozone pull the codes for free. If they can't figure it out themselves as to what the code description means (by reading the screen) and which sensor to replace to fix it (combination of bank 1 or bank 2 and sensor 1 or sensor 2), go somewhere else.

Hope that helps.
Mark

95stang5.0

well i am gettin terrible gas mileage. Like 12 mpg. .I dont have cats and i dont belive i have o2 sensors in the back. Do you think i should change the upstream ones???

WizardBlack

Yes, that's definitely a good place to start; especially if WOT power seems normal. What mods do you have?