Mitsubishi Montero Gen 2.5 O2 Sensor Replacement

jlocster

Explorer
Replacing the upstream O2 sensors.

Part #s:
DENSO 234-4738 O2 Sensor, Upstream Right
DENSO 234-4742 O2 Sensor, Upstream Left

These are the connectors. The black connector is the right side, and the grey one is the left side.
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Both O2 sensor male connectors are attached to the top rear center of the engine via metal "L" brackets and a single bolt. The vehicle wiring harness female connectors are on top, and must be removed first.
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The "L" brackets must be removed from the old O2 sensors and attached to the new ones.
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The right side O2 sensor is accessible from the top of the engine compartment.
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The left side O2 sensor is accessible from below the vehicle by the front wheel.
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You'll most likely need to use an O2 sensor socket and a healthy dose of penetrant to loosen them.
I destroyed both of mine in the process of removing them as they were so tight.

The Denso O2 sensors come with a packet of high temp anti-seize with which you'll want to coat the threads.

Be careful to keep the sensor tip clean and away from any sort of silicone.
 
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MascotRejct

Adventurer
Does your 2.5 have the 3.5L V6? I'm gonna be changing mine on my 2001 gen 3 sometime this week, and was wondering if they are in the same location...
 
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jlocster

Explorer
Yes I do have the 3.5l V6. I'm not sure if the exhaust system is the same or different on a Gen 2.5 vs. a Gen 3. I'm guessing the exhaust systems are different though, in which case there might be slight differences in sensor location. Hopefully someone who knows for sure will chime in.
 

andrewr

New member
Just finished replacing the passenger side 02 sensor. It was a hateful, terrible job to get it out. I have a feeling it was original. I ended up having to break the top off of the thing and put a deep 22m socket on it. The 7/8 o2 socket was rounding off the edges.

Be especially careful when you finally get the thing to move. I was so excited after a couple hours of smashing my arms up that I pulled a little too hard and rounded off the threads in the hole. A thread chaser fixed it but there were a few tense moments.

Hoping my mpg hops up a bit
 

plh

Explorer
I used a "crows foot" style O2 sensor socket and Kanolabs - Kroil - all 4 in my '05 came right out. Maybe an hour to do all 4. Hardest thing was to get the connector un-done in the tunnel sorta above the transmission on the driver side. (LHD)
 

andrewr

New member
o2

I used a "crows foot" style O2 sensor socket and Kanolabs - Kroil - all 4 in my '05 came right out. Maybe an hour to do all 4. Hardest thing was to get the connector un-done in the tunnel sorta above the transmission on the driver side. (LHD)

4 in an hour sounds like a dream. The 98 Gen 2.5 doesn't have a lot of space to work in on the passenger side O2 or maybe I just need to hit the gym.

The driver side looks like I'll be able to get a breaker bar on no problem so hopefully easier than the passenger side.

I love older vehicles, and having to work on the street in Brooklyn makes it especially special. I did manage to find a parking spot with a tree next to it which allowed me to brace against when turning the socket.
 

All-Terrain

No Road Required
I thought I read somewhere that of the 4 O2 sensors on a CA truck, 2 of the sensors are primary (have more tuning/adjustment authority to the ECU) and 2 are secondary. Can't remember where I read it or which sensor is which. Any info?
 

Salonika

Monterror Pilot
Good question All-Terrain, I've wondered the same thing. Even the FSM doesn't cover the logic in the ECU. My truck is CA exhaust. I've always thought the upstream (pre-cat) sensor was the one the ECU would listen to for tuning the mixture, and the downstream sensors (post-cat) would just be confirming to the ECU that the cats are doing their job.
 

plh

Explorer
Pretty sure the answer is yes. My gen 3 for sure yes. 4 different part numbers due to wire harness length. They are all different lengths.
 

rahee006

Observer
my O2 sensors are stuck tight, Took it to the mechanic since i couldnt take it off myself, he torched that thing, still was stuck in pretty tight. Should i Helicoil it or just get the Y Pipe where the downstream sensors sit?

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jlocster

Explorer
Did the threads get destroyed? If they did, helicoil should work. You might want to try to chase the threads first to see if they can be salvaged.
 

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