Nissan Repair: 1998 Nissan Quest XE: Service Engine Soon Light, nissan quest xe, car repair shop

Question
Dear Von,
The "service Engine Soon" light in my 1998 Nissan Quest XE was on when it reached 90,000 miles.  I did not bother to check it since the manual said that it would be on when it reached 90K and 120K miles.  However, since I need to do SMOG test for renewing the license plate, I went to a mechanic to reset it.  The mechanic did it for a small fee, but did not guarantee.  After the reset, I drove it back home, but in less than 1 mile, the light was on again.  Recently, I visited a car repair shop for the diagnostic ($45 charge).  They pulled the codes and told me that the car needed Oxygen sensor (front) and Nock sensor (I believe it is Knock sensor).  They charged me for labor (~$200) and parts (Oxygen sensor (~$130), Nock sensor (~$160), and 2 gaskets ($25 each).  After they finished, I came to pick it up in the afternoon (~5:30PM).  I saw the lead mechanic test-drove the car around.  Then, he told me that it was ready, and the light has been reset.  He told me to drive at least 50 miles before smog test.  I was ready to leave and turned on the car.  Surprisingly, I saw the "Service engine soon" light turned on again.  I called him and he reset the light.  On the way home I stopped the nearby shopping center to turn off the car and turn it on again to repeat the process.  I have driven only 1.2 miles.  I was shocked to see that the light was on again as soon as I turned on the engine.  I then came back to the shop.  Since it was late the lead mechanic told me to come back again next week to rescan.  He said that right now only 1 code appeared (Oxygen sensor).  Do you know what's wrong?  Did the mechanic replace the right parts?  Please help me understand more.  I was thinking to bring the car to Checkers or Autozone for free code reading as you mentioned in your answers to other check engine light questions.  Note my Quest has 100,600 miles now.  Thanks in advance for your help.  Best regards, Jaya

Answer
Jaya,

The oxygen sensor problem is not the fault of the mechanic.  They were attempting to save you money and not leasd you to think they were going to take advantage of you.  The problem comes from the error code and it only showing one oxygen sensor being defective.  The issue is that the computer reads the oxygen sensor, there are three of them.  One for each side of the engine and one behind the converter.  The two up front read each bank of cylinders and adjust the fuel for each side.  The one behind the converter reads the output to see if the fuel management is doing what it is supposed to to get you the best mileage and keep pollution down.  The real thing is that they should be replaced every fifty thousand miles for optimal performance.  They get coated from the by products of the exhaust and get lazy in their performance.  Mileage decreases as a result until the computer reads one as an error.  You replace the one that was in error and then the balance gets off and then the computer throws another code for the other one until you end up replacing all three.  If it were me I would have all three replaced.  So, you have two to go.  Again, the mechanic was only replacing the parts that showed to be defective at the time.  I have found that I just replace all of them and I am done with it.  But, at over $100 each, plus labor, people freak out that you are trying to take advantage of them.