multiple ECU codes

About 4 months ago, my 95 Altima's check engine light came on. The codes turned out to be O2 sensor bank 1 sensor 1 and Knock sensor. So I changed the O2 sensor before the cat. The check engine light remained off untill a few weeks ago. This time I got multiple codes;

0136 O2 sensor bank1 sensor 2
0325 knock sensor
0130 O2 sensor bank1 sensor 1
0400 EGR flow malfunction

I really don't know if these codes are correct? I just changed the front O2 sensor, how come it reports there's a problem with it again? I already checked the tubes that comes out of the EGR valve and it looks fine, no cracks, no blockages. What really goes wrong with my car? :banghead:

Thanks for sharing your knowledge!

Disconnect the batter for 15 minutes both terminals and drive for a while and see if it comes back. If it does then you may have a problem but you might just have bad gas for now.

You might put a gas additive in the tank also. Have you changed the air filter lately? :uhoh: