Working on moms 2000 3.0. Felt rough. Pulled codes and came up with PO153 (Bank 2 Sensor 1) & PO430 (catalyst system efficiency below threshold (bank 2)). So I did the O2 sensor. No response. Took to buddies shop and found out three cats were plugged. That was $1100. STILL no good. Then Pulled two codes; PO301 (cylinder 1 misfire) & PO 314 (single cylinder misfire). Replaced plugs; no response. Did wires; no response. Just pulled codes again and getting PO174 (system too lean bank 2), PO171(system too lean bank 1), & PO401 (exhaust gas recirculation flow insufficient detected). WTF!?!?!?

 

have you tired looking for a vacuum leak? if there is air coming in after the MAF sensor then that air coming in is unmetered. the car will be thinking its getting 11g/sec of air when its actually getting 31g/sec (just making up numbers for an example) so then the car will be running lean because the air/fuel set up will be for that 11g/sec and not 31.

now that can cause a bad reading on ur bank 1 sensor 1 and bank 2 sensor 1 readings because all the first sensors really are is just a air/fuel sensor. and because the car is running lean because of the possible leak its thinking the sensors are bad and at the same time that will some times tigger a faulty cats.

now the miss fire. if the air/fuel is too lean it can also cause a miss fire because theres not enough gas to make combustion.

and the EGR is trigging because of the possible vacuum leak.

 

Because it's cheap, the first bit of engine maintenance I'll do whenever I buy a used vehicle is replace any and all vacuum lines. While they may appear to look good, the rubber gets soft, and can callapse, or there can be small cracks that are unseen.

I'd attack the fuel/air mixture. Look at the large intake tube that runs from the air filter box to the throttle body. Inspect it carefully for cracks. Check ALL the lines that connect into the intake; these are where unmetered air can enter the intake and cause that rough idle you mention. Check the injectors themselves. A clogged injector will cause a lean mixture. Search here for ways to test the injectors. RonD has written up some great "how to's" on this subject.