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.

Pauline

teeceeum

The knock sensor detects preignition and adjusts the timing of the spark to correct it. What you hear with preignition is a knocking or pinging sound. Sometimes it's so subtle that you can't detect it with your ear, but the sensor can. If preignition is allowed to continue for any length of time it will eventually destroy the engine. So yeah, I'd say it's pretty important.

I have no idea why the two would go out at the same time. They are totally unrelated and completely separate from each other. Just a sad coincidence, I guess.

jasondevo

I don't know if it's a coincidence. I took my car in this morning (in the bay area, cailfornia) as well, 1995 Nissan Altima with 118,000 miles. Check engine light had come on. Both the oxygen sensor and the knock sensor need to be replaced. Talk about expensive parts.

labman

The computer diagnostics are not all they are cracked up to be. Quite often the codes are wrong. Unfortunately in most cases to identify the real problem and fix it requires more sophisticated equipment than they used at Auto Zone. You need to have read outs of what the various sensors reading. This means a trip to the dealer or one of the better independent shops.

Before doing that, you could check some things such as the spark and fuel injectors. A compression check might help too. A problem with fuel delivery or ignition could be causing both sensors to be giving accurate, but out of range readings and cause the codes to be set. Fix the problem, clear the codes and the problem is solved. You could go to a junk yard and pick up some spare parts, various sensors and ignition parts and try swapping things in and out and see if anything makes any difference.

$400 sounds like a lot to replace an oxygen sensor. I would be quick to try an independent before going to the dealer again.