Need help with check engine codes.

[BOWHNN]

P0171 pd - system too lean bank 1
P0174 pd - system too lean bank 2
P0402 - EGR flow excessive

When my check engine light was first on i changed both of the bank 2 oxygen sensors and erased the codes and now the light came back on.
At first the codes were saying the bank 2 O2 sensor 1 was bad so i went ahead and changed both of them and it still says those 3 codes and i dont know what to do about any of them. i need help.

 

[JohnJ]

There's a lot of things that can case the 171 and the 174 but you need to ignore them until after the 402 is dealt with. If you've actually got excessive EGR flow if can cause an incomplete burn and thus a false lean reading can result giving you phantom lean codes. You need to fix the 402 first, then clear the codes and see if the 171 and 174 come back.

For the 402, you're probably looking at a bad DPFE sensor, but it could also be an EGR valve or solenoid sticking. If you have access to a scan tool that can read PIDs as well as codes, do a quick test of the DPFE sensor. If not:

First disconnect the vacuum line between the EGR solenoid and the EGR valve and use a vacuum gauge to check for ANY vacuum from the solenoid with the engine idling. If there is, then replace the solenoid and clear codes. Otherwise:

Either the EGR valve or solenoid could be getting stuck at times. Replace the DPFE sensor and see if that fixes the p0402 as these sensors tend to go bad a lot.

If it isn't fixed replace both the solenoid and the valve.