ok, i pulled code po138 and po139 which should be downstream o2 sensor. i went to buy the sensor at an auto parts store and the guy told me the aftermarket sensors don't work. the dealer quoted me 335 to replace it. i want to save money and do it myself. when i called the dealer to just get the part, he said there are 4 different ones each are $135. there are 2 upstream. one on the cat and one after the cat. i know the one is need is one of the 2 latter, but i need to make sure. so, does anyone know for sure which one i need for the codes i listed? thanks!!! also, i know i can get a bosch off the internet for like 90 bucks. will it work?

 

Is your Maxima California emissions or not?

If is NOT Cal then you need only one sensor downstream (which is after CAT)
Bosch 13278

If is Cal spec then you need the Right Downstream O2 Sensor.
Bosch 13374 (rear - after CAT)

Both of them you can find them on-line for around $80. If your car has around 100k then I would replace them all because they start to fail anyway. I actually replaced them all on my Max ($347 )

 

thank you for your help. i was beginning to wonder if anyone was gonna respond. i ordered the one you mentioned as my car is not calif. emissions. it should be here in a few days. 79 bucks from 02 sensor wholesale. i am not changing the others because my car is for sale and i don't want to put more money into it. now, will the CEL go off when i replace or do i have to reset it? i tried the reset on the pcm but my big *** can't get in there to turn the screw. can i just unhook the battery? if so, for how long? and will it run crappy for a while or will it be fine. don't want test drivers to think it is a piece of ****.

 

i too will be replacing all my sensors soon, my check engine light came on for the rear sensor so i figure with a 106k i might as well replace them all. i think i calculate somewhere around a total of $250 off oxygensensors.com for oem bosch sensors.

 

You have to reset the CEL. If you cannot from the ECU then go to an autozone, borrow the ODB2 reader and reset it from there. Disconnecting the battery (some people do it over night - I don't know) will reset the computer (all the data that it learned), but I don't think it will reset the CEL.
I am not sure how long it will run crappy after disconnecting the baterry. Maybe someone else knows.

 

I drive a 1999 SE with 83,000 miles and I'm going through the same thing. Just two days ago, I was climbing a hill on the highway and my CEL came on. I did a diagnostic and got two codes which were both undefined. First I changed my air filter (it had been a while), assuming the engine was being starved of air on the steep incline, and cleaned the throttle body again, which wasn't so dirty.
I reset the CEL but it came on again today merging into highway traffic. I did the diagnostic again today and got two different codes...this time one was undefined and one indicated a problem with the rear O2 sensor. The CEL even started blinking after this which, according to the user's manual, is even worse than having the light stay on. So I reset the CEL again and I'm staying away from the highways for a few days while I order a new O2 sensor (or all four of them).
Do you all recommend Bosch or will any brand do? Prices on oxygensensors.com looked pretty reasonable.

 

I also have the same problem. I drive a 1999 Maxima with 73000 miles. The CEL light came on today. It has a po138 code. I also smeel a weird smeell after it is turned off in the garage. I am thinking maybe a Catalyst Converter issue.Can someone help me with this issue?

 

Damn all you guys makin me scared of my 99...oh wait all of mine are gone hahahaha but all 99s as far as i recall are cali emissions... i havent seen one none cali 99 yet and thats about 1000.. just between mass rhode island and ct...

 

Is this problem usually the O2 Sensor?

 

Disconnecting the battery for 6+ hours will turn the cel indicator off and reset the codes but it will not be ready for emissions and the light will come on as soon as it finishes the diagnostics (50 miles or so)