Pontiac Repair: 90 pontiac sunbird, pontiac sunbird, oxygen sensor

Question
i have the 2.0 4cyl non turbo about a month ago it started to bog down when i gave it gas and would barely stay running. now it will not start i have changed fuel filter and spark plugs. i dropped the tank and check the fuel pump i have also checked coil and timing chain everything is fine. when i advanced the timing on the distributor it seemed to fire better and tried to start but still wouldn't start. I have also jumped the computer and it gave me codes for oxygen sensor reading rich an oxygen sensor reading lean and the egr circuit. this car is a throttle body. i have no idea what it could be any and all suggestions or ideas would be appreciated thanks.

Answer
Hi Aron:

those older cars were pretty hard to figure out. The fact that it has both rich and lean codes is odd. I would look more tat he lean code as an issue more so then the rich code. Does this run okay cold. but not okay hot? if so I would look at possably a bad O2 sensor or a fuel pump. It may not have enough pressure and or volume to keep it running properly under a load. That is where I would start looking. good luck :-)