Emissions on a 2000 4R LTD
 

When dealing with a code PO171 - referring to the O2 sensor or MAF sensor - and aRunner that has CA Emissions, is it generally the best to change out all four of the O2 sensor's? I have been having this code now for about 6 months. I changed one of the fourO2 sensors(The one you can see looking from the passenger side wheel well), cleaned the MAF sensor and have also seafoamed the engine. I have taken it in and reset the computer each time I try a new fix, but it still keeps coming on, throwing the same code. I'm at my wits end with this. Does anyone have any suggestions on what else I can do before I have to break down and take it into the dealership?