I have a 96 Nissan Altima with a mileage of 130,000. The Search Engine Soon light is on last night, so I check with AutoZone today, the wrong code is PO325 and PO420, representing knock sensor and oxygen sensor respectively. The light was on once three years before and I have changed the Oxygen sensor from the dealer for $400.

So my questions is:
1)Why there are two wrong codes given the Search Engine Soon light was on from yesterday, the two sensors are wrong simultaneous?

2)How serious is the problem with the Knock Sensor? The guy from AutoZone said it is fine, just a sensor, while some article from the Internet mentioned the knock sensor is an important part for engine?

Thanks a lot.