Nissan Repair: 1998 Nissan Sentra SE, nissan sentra, oxygen sensors

Question
My service engine soon light came on yesterday so I took it to autozone and the car had a P0135 code. This is a O2 sensor Bank 1 or something like that. The car has 2 O2 sensors 1 at the exhaust manifold and 1 after the cat, Is this code for the 1 at the manifold or the 1 at the cat. Thanks in advance for the info, John

Answer
John,

Oxygen sensors are really supposed to be replaced every 50,000 miles but no one ever does.  I have 135,000 on my Maxima and still have the originals.  The oxygen sensor you need to replace it the one closest to the engine.  The one behind the converter is only to rat on you if you remove the converter as it is only looking for a reference signal.  Change the front one and disconnect the battery for about five minutes and the code will reset and you should be good to go.  Basically, what happens, is the oxygen sensor gets coated with contaminates from the fuel and over the years and miles it gets coated to the point where it no longer functions properly.  In the trade mechanics call them lazy since they are coated and cannot read the exhaust mixture from coating on them.  Eventually, they set a code and you end up replacing them.  The nice part is your mileage will increase after you change it.  Currently, with the old one, the coating makes it read lean and that causes the injectors to run rich.