Nissan Repair: Codes!!, oxygen sensors, oxygen sensor

Question
Hello,

My 2000 Maxima (130k miles) recently passed its emissions test. However, it did detect 2 DTC (diagnostic trouble codes)

P0138
P0139

Can you please explain to me what they stand for and how urgent should I look to have it repaired.

Thank you,

Answer
Hector,

Replace the oxygen sensors.  There are three.  Two up front and one behind the converter.  I have a 2000 Maxima SE and have replaced mine at about the same mileage.  I have over 150k miles.  The sensors get coated with exhaust by products.  I would replace all at once and be done with it.  A good thing is the mileage gets better with new oxygen sensors.  rockauto.com or courtesyparts.com are both good sources for the sensors.  The dealer is twice the price.  You can do it yourself and save even more.  You will need to invest in an oxygen sensor socket.  Disconnect the battery to clear the codes when you are done.  Get them replaced soon as your mileage is suffering as a result...