Toyota Repair: 1997 Toyota Camry check engine light error code P0420, toyota camry xle, napa auto parts

Question
I have a 1997 Toyota Camry xle 3.0 v6 engine and I was getting the P0420 error code, and took my vehicle to a muffler shop and had the catalytic converter and the long pipe that connects to it changed with a universal catalytic converter. The pipe was changed because it had several holes and was making the car really loud. I had the code cleared after that was done and about 2months later the code came back on and I was told by Napa auto parts to have it cleared again and I did; and if it came back on then I needed to replace the downstream O2 senor; and the check engine light did come back on reading the same code PO420. But I have read lots of different reviews on line and I am very confused now.

What should I do? Should I change both O2 sensors or just the downstream? Does this year model have more than one catalytic converter? Was it wrong to let the welder put a universal catalytic.

The only thing the car is doing wrong is drinking lots of gas, I am getting like 17miles to the gallon (awful).My mother did purchase this car used about 8 months ago but in the last month we have changed the spark plugs and wires, air filter, PVC value, oil and oil filter, and gas mileage still has not improved

Answer
The P0420 code is a catalityc converter inefficiency code and is not an O2 sensor code, it actually has two catalityc converters one right under the exhaust manifold and the one in the pipe that was already replaced, I would recommend replacing the front converter that attaches to the exhaust manifold.