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DIY: Do It Yourself
Post here to share or improve your wrench turning skills! All BMW E46 DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

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Old 12-24-2010, 10:11 PM   #1
Nervous
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DIY: Post-cat O2 (oxygen) sensors replacement (w/pics)

I've done post-cat O2 sensors replacement today. Took me about 3 hours total, but I wasn't in hurry and did everything slowly, to make sure I do not screw up anything.
First of all, remove cabin air filter housing and decorative top motor cover with "BMW" on it. Jack up driver's side of the car (it was enough for me to jack up only the driver's side). Secure the car safely on jack stand(s). Remove plastic protection pan from underneath. Remove aluminum reinforcement plate from underneath as well (if your car is equipped with one):


Now, unscrew the sensor that is closer to you (let call it "front"), it has easier access:


It is a good idea to mark it first by a marker. Just put "X" on it.

So, to remove this sensor I've bought an O2 sensor wrench with tilting head (22mm). It is very convenient, but I found out that regular 22mm open-end wrench works as well:



You may need to use a heavy hammer to convince O2 sensor, just be careful not to snap it. If you use the O2 sensor wrench then you might need a piece of pipe to increase momentum. So, unscrew it a remove from the pipe.

Another senor (let call it "rear") is a bit trickier. O2 wrench does not work for it since there is no enough space there. However, again, a regular open-end 22mm wrench works fine there:


Hammer (I used 4 lbs one) is also useful here to loosen the sensor. Remove it from the pipe:


Now, mark the connectors. Connectors for post-cat sensors are located at the right side from the valve cover if look at it from the front (so, closer to driver's side):


Mark connectors, so, you'll be able to determine which one is which. The connector for the "front" one (that we marked by "X") is located closer to radiator (easy to remember: "front" in the pipes - front in the connectors).
Disconnect them, make sure wires are free (I also needed to remove a couple zip-ties to release the wires). Go under the car and pull wires one by one. Here are removed sensors along with new ones:


Now time to put new ones. First of all, mark connectors as "rear" and "front" - might be useful when you'll connect the connectors.
Start to screw in the "rear" one, then the "front":


Make sure wires are not twisted too much during the procedure. Use 22mm regular wrench to secure the "rear" sensor, and either the same wrench or O2 sensor wrench for the "front" one. It is impossible to torque them by torque wrench there, so I just tighten them by hand, making sure I do not over-tighten them. Good enough for me.

Now the fun part - fishing for wires from top. You need to pull connectors to the top of the engine. I used a rope taped to a metal stripe, which was removed from the old windshield wiper. Being on top, push the metal stripe with the rope down to the sensors. Go under the car, tape in a connector for the "rear" sensor. Push the stripe back, to the top. Go back to the top of the engine and pull the rope and the metal stripe with the sensor:


Repeat for the "front" sensor. Connect connectors ("rear" - to the connector closer to the firewall, "front" - to the connector closer to radiator).
Job is done.


Put back everything (reinforcement plate, plastic motor protection, etc) and go for a test drive.

Last edited by Nervous; 12-24-2010 at 10:14 PM.
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Old 12-25-2010, 09:03 AM   #2
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Very nice!
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Old 02-24-2011, 01:59 AM   #3
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Very nice thread! A+

I am doing my o2 sensor now. I have the universal bosch one.... does anyone know if i just match the color's together? The o2 sensor i got need the connector piece added on.
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Old 02-24-2011, 06:18 AM   #4
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Well done DIY.
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Old 03-07-2011, 06:02 PM   #5
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Starting to troubleshoot

Hi,

Just scanned my OBD and got those 3 codes:
  • P1181 O2 Sensor Signal Circuit Slow Switching From Rich to Lean (Bank 2 Sensor 2)
  • P1183 O2 Sensor (Bank 2 Sensor 2) Open Circuit During Coast Down Fuel Cut-off
  • and P1397 Camshaft Position Sensor "B" Circuit (Bank 1)

For now I'll focus on the O2 sensors (CPS will be replaced).

I took a ride to log the activity of the 4 oxygen sensors, and here is an extract of 1 minute of city driving shown as a graph.

Few questions:
  1. Which one is the Bank 2 Sensor 2? pre or post-cat?
  2. I thought O2 sensors always fluctuated between 0.1V and 0.9V, but I see between 40 and 50 seconds on the graph that they went to 0.0V. Why? Should I worry?
  3. Should O2 sensors be replaced by pair?

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Thanks a lot!
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Old 03-08-2011, 08:24 PM   #6
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bump... any idea?
I erased the codes to see which one would come back first.
P1183 got thrown this morning on my way to work. P1181 isn't back yet.
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Old 03-13-2011, 01:26 PM   #7
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bump again... I can't believe no one here knows how to troubleshoot O2 sensors.
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Old 04-12-2011, 08:29 AM   #8
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El Polo...

Its not that no one knows its more that trouble shooting emissions systems is a PITA and cant really be explained in a few posts. I'll try a fast run down of the basics.

Bank 2 sensor 2 is your back Cyl 4,5,6 post cat o2 sensor.

Look at post 38 on the link I posted.

LINK

On your graph you posted, there are a few things to be looking for one is the way the post cat o2 looks while running cold and hot 1 minute doesn't really give enough information but there are two deferent patterns for the post o2 you could look for.

Cold- you will see an isolating pattern as the voltage changes up and down the system isn't hot yet
Hot - this pattern should even out as the sensor starts reading the hot air and is giving the computer the current information.

Also sensor 2 should not flux with sensor 1 that could be an indication of the cat failing.

Sensor 1's job is to talk to the computer and tell it if its adding to much or to little fuel, Sensor 2's job is to make sure the cat is burning the rest of that fuel.
As sensor one is adjusting constantly do to the ever changing conditions of the motor the cat should burning extra fuel so that sensor 2 reads an constant emission output or fluctuates more slowly.

Sensor 1 reading should be fluctuating up and down
Sensor 2 should have a slow lightly fluctuating reading

Basically my point is if you don't know how to diagnose a o2 have a shop do it, Then replace the part yourself if your so inclined.

There is allot of information on Google about the diagnosing of these problems lucky for you every car has emissions systems so there is allot of very good information, but its better to have someone with the know how of how the system works and how other parts can effect its operation.

Thats all I have to offer :/ Hope it helped a little
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Old 04-12-2011, 02:46 PM   #9
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Quote:
Originally Posted by GuitarmanTT View Post
[...]Hope it helped a little[...]
You kidding... right?
Heck, that helps a lot! now I understand much better how they work. I'm about to change the faulty sensor anyways. I couldn't get it out using a standard 22mm open ended wrench so I bought a special socket.
Thank you very much!
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Old 12-18-2011, 05:44 PM   #10
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I am bringing this post back up. I found almost no way that any tool would fit bank 2 , post-cat. I have been working on cars for years and it seems like a lost cause. I soaked the O2 connection liberally with PB Blast before , and during and after using a pry bar was able to get the angle tool (note , on my car there is absolutely NO way a 22mm box wrench will work) from Autozone in there. I still could not turn it as it hits this bank 1 pipe.

I am in no way interested in removing an exhaust manifold to change an 02 - what a crap design BMW.
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Old 12-18-2011, 06:00 PM   #11
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Hmm... It worked for me (see pics). You don't need to remove exh manifold, but you may remove the post-cat exh pipe. Still a lot of PITA tho.
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Old 01-06-2012, 03:25 PM   #12
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I think it's our 323it's RCase. There's no space between exhaust pipes for me to get at the bank 2 sensor. Even worse, I think the crows foot that I got on there has stripped the O2 sensor. It's the original sensor at 230k miles, and I think it's welded on
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Old 01-06-2012, 05:48 PM   #13
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dropping to .01v is normal and will not be picked up by the dme.
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Old 10-12-2012, 08:42 PM   #14
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Bump... Planning on doing this tomorrow! Just got my O2 sensors in from Amazon!
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Old 10-17-2012, 04:40 AM   #15
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I hope your name doesn't hold true about your size man because you'll be at a real disadvantage. This job sucked. What some of these guys say are true. Incredibly difficult to get at the o2s. The one more towards the front of the car was a little easier. It's that back one that really sucks. Even if you get something back there and on to the o2, there's so little room to turn your ratchet, or even finding one thats almost the perfect length and height to do that job. If i was to do this again, I would try using a 22 wrench, but also, I would seek out a pivoting ratchet with multiple extensions of different size. That or an adjustable one. If i had that extension with maybe even a u joint at that time, I think I wouldn't have had such a hard time. Since you're also on your back and shaking your car around as you attempt to crank that thing loose, i recommend sturdy jack points, with maybe even your rims laid down for extra security. Also, find something for leverage. I was using the long metal jacking stick from my jack. Worked wonders.
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Old 12-15-2012, 06:23 PM   #16
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must be the 323's

i could not get at my rear post cat 02 on the 325 until i pulled the skid plate.. then I had to remove a bolt but thats only bc someone bolted the manifolds back up without using OEM hardware and they were too long.

either way, the post cat 02's I have the same number stamped on them and I did not differentiate when I pulled them. I hope to hell they are the same part bc I am just slapping back in there and running with it. Anyone know?

at least I can plug the front one into the front connector per this thread... nice read.
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Old 12-15-2012, 06:39 PM   #17
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Two rear o2 sensors are identical. You just need to make sure you connected them into correct sockets.
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Old 12-15-2012, 08:40 PM   #18
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cool... i am connecting them like you said above.. the rear sensor to the rear harness location. However, apparently I bought parts that have a slight variation in the connector. The are almost opposite. I am in the process of chopping the connectors off the old sensors and hardwiring them in. what a pain..
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Old 12-15-2012, 09:14 PM   #19
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What a pain right? And that's on top of of having to get them off. I almost gave up
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Old 04-26-2013, 10:33 AM   #20
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Can you just disconnect the O2 sensors and not use them? Clip them loose and take out the wiring? Or will this throw some error that prevents the car from running? I'm getting the 30 Amp fuse blowing that puts the car in limp mode.
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