chinatta

My car is 1998 Sentra SE. Recently the engine light is on. I went to autozone, checked the light and got the P0130 and P0325. They said i must change oxygen sensor and knock sensor.
Other problems are that my car has very bad acceleration and poor MPG (~20). So I think it is really necessary to change them even though they cost ~$300.
Questions are

1) which oxygen sensor should I change? Before catalyst or after catalyst?

2) which brand of oxygen sensor is better? most of oxygen sensors sold online and autozone are Bosch. I heard that Bosch has a little bit higher voltage than OEM part, then it will burn out soon. I am not sure the comments are right or wrong.

3) where do you buy these parts? which online shop is reliable?

Thanks

KUMADE8

Don't always believe codes. They are only circuit problems not always part failures. If it is a 02 heater code then chances are the sensor is bad. Some times 02 heaters are fused, so inspect for a 02 heater fuse. Use only Bosch, Denso or OE sensor only if they are bad. They post 02 can cause very minamal driving concerns. The pre 02 is the main one. Common problems for power loss, plugged fuel filter or low fuel pressure. Air flow meter wire contamination, which can be serviced. What where the code discriptions.

JDMuscle51

P01 codes are basically ignition miss-fire codes, that could be as simple as a bad spark plug, plug wire, or it could be jacked up coil pack.

KUMADE8

PO301 TO P0310 ARE GENERIC MISS FIRE CODES AND LAST 2 NUMBERS RELATE TO CYL.# WHEN THERE IS A P1301 THAT WOULD BE A FACTORY CODE FOR MISS FIRE AND AGAIN THE LAST 2 NUMBERS RELATE TO CYL.#