2004 Mustang P0175

Hello, I am new to this and I still don't know yet much about cars.
This past week I got the service engine light on. I checked the codes which were P0171 & P0175. I put back on the original stock air intake it removed the P0171 code. Thought the P0175 code was because of the o2 sensors so I changed all 4 sensors. The service engine light turned on again with same code P0175 too rich bank 2. Any suggestions what it could be? Its a 04 V6 Mustang.

Yes I did but light keeps coming on. There is also a nasty rattling sound from the driver side catalytic converters. I went to a mechanic to see what was the problem and they said that its most likely the reason since they sound clogged/messed up. He is charging me around $450 to change both from driver side. Not sure if that's a good price?