Hello! I am working on a 99 Altima with 182000 miles on it. It is not my car so I know virtually nothing about the car's history. The engine light is on and it idles rough. I am getting codes P0400 (EGR flow malfunction), P0325 (knock sensor circuit malfunction), and P0171 (system too lean). This car belongs to my son's fiance and she doesn't have much money so can't really afford to throw parts at it, so hoping that someone might have some input as to where to start with the diagnosis and any tips for diagnosing this. Thanks!

99 altima p0400 egr malfunction: this usually means its stuck or full of carbon, alot of this carbon from the fuel grade used more so in winter because of the ethanol (alcohol) sewems to delvelope on the valve system around the erg valve. clean /replace. the other two
code will more than likely go away because the recir system will restore air to return hot gas to engine balance. sky5031