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Hello everyone.
I have a 2001 Honda Accord Ex Vtech, F23A1 with a bad upstream O2 sensor. I have the new Denso sensor. I went to put Mouse Milk on the sensor today so I can loosen up what is certain to be a tough removal and discovered my sensor is inside of the exhaust manifold. About 1/2 of the sensor is inside the manifold. I cannot access or see the hex bolt part of the sensor. It is buried in the manifold.

All of the videos online show the sensor completely outside of the manifold up to the bolt. Is this normal? I cannot get the sensor removal tool I purchased inside the rather small hole in the manifold nor can I get a standard socket in there. How do I get the thing off? DO I have to remove the manifold? There seem to be 3 bolts on it, all rusted but saturated with mouse milk now. I cannot cut the wires as this is my ONLY car and it needs to be driveable should I have to take it in to be repaired but I should be able to do this job on my own.

The engine block was replaced about eight years ago because the shop allowed carbon to get into the engine when doing a repair. Is it possible they put a non-standard manifold onto it? Every picture and video I have seen shows the O2 sensor mounted on the outside of the manifold ...

Ideas?



OK. So my cousin said it was the exhaust manifold but it may be the header? The O2 sensor that needs to be replaced is in the engine compartment, not near the cat. I may have the terminology incorrect in my above post. Cousin works on US cars and he seems to be confused ....
 

· 2020 best year
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Definitely doesn't sound right, but photos would help. I think maybe @enne can answer this more precisely.

Also, you can temporarily run the car without any input from front O2. Your car will just run in open loop mode. Not optimal, but it will run.

Lastly, exhaust manifold and header have the same purpose(and these words are thus used interchangeably). Exhaust manifold usually refers to the OEM piece, while headers are used to refer the aftermarket options that are performance oriented.
 
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· Runnin' in the 90s
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@UnknownJinX is correct with the terminology, the OEM exhaust manifold is referred to as such, and an aftermarket exhaust manifold is usually called a header. So no worries there. My 1999 Accord had the OEM exhaust manifold replaced with an OEM-spec one. My Camaro had the OEM exhaust manifold removed and upgraded headers installed. Camaro now sounds angrier. lol

So, F23A1 is the non-ULEV version of the 4-cylinder engine. Mine is a F23A4 so the design may be a little different. Let me go take some pics of mine and check my Haynes manual and see what it says and if there's a difference. I know the upstream one on mine, sticks right out the front of the exhaust manifold, through the heat shield. I'll update this post with pics.

Does yours have the heat shield on it, or are the pipes in plain view?
 

· Runnin' in the 90s
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First two photos are of the manual which shows the A1 if I'm not mistaken. No instructions on how to remove the upstream o2 unfortunately. But it does show how to get the heat shield off so you can see the manifold.

Next three photos are of mine, with the heat shield over the manifold. And yes there is a considerable amount of rust in the area. Might want to take it to a shop and have them crack the bolt for you... If the o2 sensor is truly inside the manifold, and not like the photos here, they may have put a different brand of manifold on it when they changed the engine?



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Discussion Starter · #5 ·
Thank you for the pics! Very helpful. The difference between the Vtech and ULEV placement for the O2 sensor is the Vtec is on the left side.

I was very tired last night and working in a fairly dark garage and I am partially blind. The manifold cover/heat shield is different on my car. It seems to be an aftermarket one put on when the engine was replaced. The opening where the O2 sensor goes is so narrow, you cannot even get a socket in around it. So it looks like I have to take off the heat shield but it also appears to be held in by the three bolts which I have soaked with mouse milk. They are pretty rusty on the outside. Unfortunately I do not have a 1/4" ratchet or adapter for my 3/8" ratchet so I can't take them off until I get the adapter from amazon tmorrow. (The Ex took all of the tools. Every time I have to do a repair around the house, I find out what I am missing. Amazon has been my savior. I am the Mom to two disabled kids who require constant care and supervision and do not live near anything so I can't just run out and pick something up.) I am expecting to find the O2 sensor screwed into the manifold and accessible once I get the cover off. At least I hope so!

I cannot thank you enough. I am hoping to do this myself. I called around to my "local" shops (Some as far as 35 miles away.) and they are quoting me $150-$400 for labor to change this O2 sensor out. They also tell me they won't put on my perfectly good, new-in-the-box Denso sensor as they "do not install parts purchased by the consumer". So they would make me buy another new Denso sensor. I got mine from amazon for $41. They are quoting me $85-125 for the same part plus installation!

Should the cover come off with the removal of the three bolts? Anything I should look out for in taking off the cover?

Thank you again.
 

· Runnin' in the 90s
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So, it seems you have an aftermarket header and heat shield then.

It should be fairly simple taking it off, minus the rusty bolt part. If it doesn't want to come loose today, soak it again in penetrating oil, let it sit overnight and try again tomorrow. And if you end up breaking the bolts, it's okay to just leave the cover off, it doesn't necessarily have to be there. Do you have a breaker bar, or just a length of pipe, to use for more leverage?

That sucks they took the tools with them. But yes Amazon is awesome :)

A lot of shops nowadays won't install parts that they didn't source themselves. I have a shop nearby that will do it and I'm very lucky that they will do that for me, every other shop in the area will not install customer-sourced parts even if they're brand new in the box. I had my shop install a brake switch about two months ago on my camaro. I had the part but not the tools to install it.

Let us know how it goes!

What the heck is mouse milk?
https://www.mousemilk.com/
(You can get it on Amazon ;) )
 

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Discussion Starter · #10 ·
Update

Got the ratchet adapter and took the three heat shield bolts off. Had to sand off some of the rust to get the socket on but the rusted bolts came right off. They had been soaking in Mouse Milk for two days. Was actually shocked how easily they came off! They were really rusted.
Took off the heat shield and exposed the entire O2 sensor to the hex. It actually looks good, not rusted at all.
Fought with the wire clip but finally found a video that showed the clip and how to get it off. Most of the videos just show the guy snapping it off with one hand. It was a little more complicated! Put the wrench on the O2 sensor to see if I had lucked out but it wouldn't budge. Put on Mouse Milk, waited two hours, still won't budge. Put on more Mouse Milk and will check it tomorrow. Two days of Mouse Milk got those bolts off so I am hopeful! Will update again as soon as I got it all done! Very hopeful this will be a success.

Now if only I can get the drum out of my dryer so I can get to the thermal fuse and heating element and replace those.... and then fix that leaky faucet in the shower ... and then .....
 

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Discussion Starter · #11 ·
Hi everyone!

The O2 sensor is out!!!!!

For anyone who needs help with this, I used Mouse Milk, a 22mm Flat Wrench (and a cheap one at that), heat from the engine and a lot of elbow grease. I had to take off the heat shield to access the bolt on the sensor. I go that off yesterday. Squirted on the MM and let it sit for two hours. Tried the wrench, nothing. Repeated one more time last night, nothing. Put more MM on it last night. Tried this morning, nothing. Squirted on more MM and turned on the car. Let it run for 10 minutes. Got some leather gloves, turned off the car and tried, nothing. Squirted on more MM, it sizzled. Tried again and it came off!

So no special tools. Just me, a wrench, Mouse Milk and my car but I got it off!

It is a real triumph for me as everyone I talked to (except you guys) said it couldn't be done and I wouldn't succeed and I did it!

Waiting for the car to cool down to put new sensor in, then reassemble the car and turn off the CEL. And yes, I will be using the anti-seize. I am waiting because I am partially blind and I use tactile senses to do a lot of things. Can't feel much through leather work gloves!

Thank you all. Will give what hopefully will be the final update.
 

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Discussion Starter · #12 ·
Successful O2 Sensor replacement complete! Thanks everyone. And they all said it could not be done .....

PS Heating element and fuse replaced in dryer too. Faucet leak fixed. Toilet fixed. Two door frames fixed. Ceiling fan fixed. Waiting on belt for John Deere and one more faucet to go!
 

· Runnin' in the 90s
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Yay! That's great news. I'm still curious to what kind of manifold/heat shield was on there, but nevertheless you figured it out and got it done. Let us know about the o2 sensor/code after you drive it for a while.
 
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