My wife's 07 elantra engine light came on so I went to Auto Zone to have it scanned and P0171 code came up. The car was hard starting ,increase fuel usage and poor performance.Auto Zone told me top fix was mass air flow sensor.Repaced sensor car runs and starts better but light came on again with same code po171 and now car is hard starting again. Where do I look next, any help would be great.

 

It means you're leaking air.

OBD-II Trouble Code: P0171 System Too Lean (Bank 1)

Check for vacuum leaks... MAFs barely go bad. A bad MAF means your car shuts off. The most likely cause from the get-go is a vacuum leak.

Follow the intake from the MAF to the throttle and see if you find any cracks on the intake hose, and any other rubber hose that is connected to it, such as PVC return hose, brake booster hose, etc. A good way to detect is to spary soapy water around areas that are cracked and see if the bubbles are sucked in. If they are, simply replace.

I would still hold on to the new MAF if it turns out to be a vacuum leak. After a while just put your old MAF back and see if the problem comes back. If it doesn't come back I would try and return the MAF.