O2 sensor question
 

Hey just a simple question. Are all four o2 sensors interchangeable for my 2001 Mitsubishi montero limited?

thank you

No. The plugs are all different and the wires have different lengths

I know the pre cats are reading different info from the post ones, but how do they vary besides wire lengths? I’m ignorant so just curious. they look exactly the same…

also, another question. There are four o2 sensors and I understand the two pre cats are reading off the exhaust manifold output and I know one is after the cat converter but what does the fourth one do?

thank you

O2 sensors are paired 2 for each bank of cylinders. Setup is similar on driver and passenger side of the engine. Just after exhaust manifold you have Sensor 1 on each side (B1S1 and B2S1). These are pre-cat sensors. They provide feedback to the engine management computer as to how much fuel needs to be metered out by fuel injectors in order to maintain optimum gas to air ratio. After that, there are small catalytic converters on each branch of the Y-pipe. Following that are two post-cat O2 sensors (B1S2 and B2S2). These are for measuring exhaust gas quality after cat convertets and relate to emisdions control.
Typically, engine does not run at one steady level of fuel to air mix ratio, Instead computer rapidly alternates rich and lean mixture (within certain range). That way you have exhaust gas that contains pulses of slightly rich in oxygen or slight rich fuel plow oxygen). First set of sensors measures those pulses. The extra fuel gets burnt off in the cat converter and ensures that the rest of combustion process is completed (CO is converted to CO2, etc.) Since oxygen is consumed here, second set of sensors reports that data to PCM plow steady reading of O2 level). If your cat is not working well, you'll have O2 pulses coming through and thst is where you'll get Cat converter below efficie,cy threshhold DTC set.

Thanks for info. A few more questions.on my montero limited, is bank one sensors one and two on the drivers side?

since I believe these sensors are all original and I have 186,000 miles, do you recommend changing em all? Engines runs quiet and beautiful but I do have 2 codes pulled and check engine light is always on.

once again, I really appreciate your help

dan

On left hand drive Montero [like the ones in US), Bank 1 is passenger side (cylinders 1, 3, 5), Bank 2 is driver side of the engine.
It is difficult to say whether to change O2 sensors or not. Some people nearly religiously believe in replacing them as soon as you get any O2 sensor related codes. I'm more of the school - if they work and report good data, leave them along.
What kind of codes do you have showing?

Two codes:

p0170

p0173

I guess it’s the mass airflow sensor

Both of those codes are fuel trim (metering) related. It's hard to pin point exact cause without diagnostic. It could be MAF sensor related or a vacuum leak unmeteted air intake), or even fuel system - injectors or fuel pump. Don't start blindly swaping parts. Try to isolate issue, replicate conditions under which the code is set to narrow down your culprit.

I’m assuming I would need a shop for diagnostics unless you can suggest any dyi ways to assess the possibilities