On my 2007 LR3 I am getting two persistent fault codes: P0172 (System too Rich Bank 1) is most persistent. And code P0174 (System too Lean Bank 2) is significantly less persistent by still pretty frequent. I am not a mechanic and I'm very new to this, but it seems to be this might be saying that oxygen is getting a priority to Bank 1 and Bank 2 is getting a smaller portion. Obviously, the intake air does get divided between these two banks, so could this be pointed me towards what my real problem is?

 

When both banks experience lambda issues one can often point to something upstream causing the fault. I would clean the MAF sensor, inspect engine filter and maybe run some fuel injector cleaner. Also post which engine you have.

 

4.4 V8 gas. I did clean the MAF sensor (actually twice), and I just replaced the engine air filter. Not sure what to do about the fuel injector cleaner but my mechanic did just replace the #7 fuel injector. (I was scared to do that!)

 

Injector replacement is an odd thing, they typically last a very long time on the 4.4. If they have issues, they usually just need a good bench cleaning. Try fuel system cleaner. Did you verify injector replacement? The replacement would be the same yellow color as the others but cleaner looking. Make sure it is not another color, that will cause issues.