I have a 2002 montero xls and the service engine soon light is on. I ran the codes and got two. Po325 which is the knock sensor, and po155 which is o2 sensor (bank 2 sensor). I'm not a mechanical by any means but I am pretty handy and I can work on older cars but not so much with cimputers engines. So my question is could I do this fix at home and if so does anyone have any idea where I should start.

 

Can't help you with the diagnostics, but I can help you with location of the devices. Very straight forward to replace, but I don't know if your problem is that of bad parts.

Below is my experience with a 3.5L from my 2002 Montero Sport XLS AWD:

The knock sensor is a small black chip about the size of Chicolet piece of gum and is on top of the engine to the right of the oil filler.

Regarding the "Bank 2 Oxygen Sensor", I don't know which bank is number 2 (drivers or passengers?). The drivers side (left bank) catalytic converter has an O2 sensor upstream of the cat as does the passenger (right bank) side catalytic converter.

The passenger converter has the downstream O2 sensor for the right bank. At the end of the converter pipe is a wye with a flange. The left bank downstream O2 sensor is located there.

I bought Denso O2 sensors from Rock Auto. Buy the Lisle O2 socket which has a sidewall slit to allow the wire to pass. UL = upper left, DL = downstream left, UR=upstream right, DR = downstream right.

The upstream O2 sensors connect to sockets at the top rear-right of the engine. The downstream O2 sensors connect to sockets on the right frame rail at about the door post location, just forward of the muffler.

 

I would recommend NOT jumping straight to replacing parts. Having a code does not equal to having a bad part (sensor). It simply means that computer detected readings which are outside of the expected parameters for a specific circuit. This could be due to a bad sensor, or due to a chain reaction down the line from some other failed/malfunctioning component. If you really want to do the work yourself - look at the Sticky Posts on the Montero page. Locate manual for your truck and start reading. Follow the diagnostic and testing procedures for the codes you have and see if you can figure out why you are getting them.
Speaking from experience - Montero parts are expensive (even aftermarket). As a result, a blind shotgun approach at swapping everything in site due to a code is a costly way to go.

 

Thanks for the info. I had it diagnosed. They said the knock sensor is bad and the left upstream o2 sensors. They want $672 for the work. I think I'm ganna try to do it myself.

 

Well if they have confirmed the sensors are bad, then go for it.

The left upstream O2 sensor will be a little involved as access is not great with the converters in place, put nonetheless you can do it.

If the knock sensor is bad (and if I am right on the black chip being it), you can replace it in about 5 minutes or less. I googled for the sensor and the images on the Rock Auto website look nothing like mine, so not sure who is wrong (me or Rock Auto). I have torn my engine completely apart and I have not seen a device that looks like what Rock Auto shows.
2002 MITSUBISHI MONTERO 3.5L V6 Knock (Detonation) Sensor | RockAuto


Separately, I googled for the knock sensor location and found an image (reportedly from a full size Montero) that shows a knock sensor below the intake manifold and above the water pipe (this will be involved to replace, but not horrible). I can tell you in no uncertain terms, that my 2002 3.5L in my Montero Sport does not have this. This thread Search | Monterosportonline.net Forum states that US Montero Sports do not have a knock sensor, so if that is the case, I do not know what the black device is that I had assumed was the sensor.