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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 11-26-2005, 03:40 PM   #1
r0x
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Pre-cat Oxygen Sensor DIY (lots o pix)

I couldn't find any good DIY for this job, so I figure I'll post my own. It's not terribly difficult. I estimate that it should take the average person about 1 hour to do.

First a little background: my car was very hard to start yesterday, which hasn't really happened before. After starting the engine, it ran very badly for a few seconds, clearly misfiring repeatedly, and then it stalled. It did this twice, and finally after the third start, ran normally.

My Peake code reader displayed the following codes:

Pre-cat oxygen sensor voltage, Cyl #4-6
Pre-cat oxygen sensor heater circuit, Cyl #4-6

Plus a few random "Misfire detected" codes.

So clearly, the second bank pre-cat O2 sensor had died. For what it's worth, my car is a 2001 325Ci with 77,000 miles.

More background: your E46 has four oxygen sensors (also called "Lambda probes"). They are all plugged into the exhaust, and assist the car's computer by constantly running a kind of "urinalysis" on the engine; they measure the chemicals in the exhaust, thereby giving the computer information on how the combustion process is going. This is good, because it aids in efficient running (power, gas mileage) and can help diagnose other problems if the car is running poorly.

But oxygen sensors often fail before the other components, and because they're expensive (~$150US), usually aren't changed until they fail, unlike cheap things like spark plugs which get changed every so often no matter what. Worse, when they fail, they throw OBD codes but don't always throw a "check engine light." So it can start going bad and make the car run like crap before you really notice it (poor gas mileage, for example). Even worse, if the sensor is giving bad data to the computer, then the computer may try to compensate by changing mixture or timing, which can cause other problems like fouling plugs or destroying catalytic converters, when nothing was really wrong in the first place!

Oxygen sensors also need to be running at a specific temperature before they give correct readings, so they have built-in heaters. It seems like my sensor-heater failed, so the oxygen sensor was giving wildly bad data to the computer while the car was cold, causing the misfires and stalling.

Two of the sensors are plugged into the exhaust manifold, before the catalytic converter, and are thus called the "pre-cat" sensors. There are two more in the exhaust pipe behind the catalytic converter, and those are the "post-cat" sensors. This DIY is for the second bank pre-cat sensor. The first bank sensors measure cylinders 1 to 3, while the second bank measures cylinders 4 to 6.

You're going to need a special tool to remove the oxygen sensor. It's available at any car parts store, and usually costs between $10 and $20US. It is universal in size, so it's not peculiar to your BMW. It looks like a spark-plug socket, with a long slit in the side:



The only other tools required are: a torque wrench, a ratchet wrench with a long extension, a 10mm socket, a T25 Torx bit, a small flat-head screwdriver, and two cups of hot cocoa (it's cold out).

The exhaust (and thus the oxygen sensors) is on the right side of the vehicle (left side, looking from the front of the car).



The oxygen sensors look like long chrome spark plugs, with thick black cables coming out the back.



We'll have to start by removing the cabin air filter housing, the wiring bracket, and the engine covers. First, unscrew the top of the cabin air filter housing. There are three knobs that you give a half-turn with your hand. The lid lifts off easily then:



Pull out the filter:


Last edited by r0x; 11-26-2005 at 03:45 PM.
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Old 11-26-2005, 03:42 PM   #2
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We'll have to remove the wiring harness before the housing bottom can come out. Use a small flathead screwdriver to unclip the bottom of the wiring harness, and pull it off. Pull the wires out, and let them hang.



We're ready to remove the bottom of the cabin air filter housing. There are four Torx-head screws. Use a T25 sized Torx bit to remove them, then just pull the housing out of the car. Take a little care to note how the wires and gaskets are routed around the edges, so when you replace them later, you know how they go.



Now remove the engine covers. This section is kind of optional, but I think it gives a little more room to work, and plus I had my camera ready. Use a flat-bladed screwdriver to pry off the four bolt covers. Try not to drop them into the engine.



The four bolts underneath (actually, two bolts and two nuts) are 10mm each. Undo these and remove the plastic engine covers. Once the engine covers are off, I like to put the oil filler cap back on because you don't want to accidentally drop a sock down there or something.



Locate the oxygen sensor you're trying to replace, then follow the wire upwards a little bit. There's a silver clip holding the wire. Just pull the wire out of there.

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Old 11-26-2005, 03:43 PM   #3
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This next part is complicated. Follow the wire along, until you see this connector. Just pull it out of the big bracket (part A). Then unclip the two tiny clips on the "boot," then open the boot like a clamshell and remove it (part B). Then, on either side of this connector are two tiny clips (part C). Pry them open, then just pull the wire out of the connector. This whole step takes patience and fine finger-work.



Here's a close-up of the "boot" clip:



The wire should be completely free now. Slide your oxygen sensor tool all the way down over the sensor, threading the wire out through the slit in the side. Then plug a long (12 inch or longer) extension into the top of the tool, then plug your ratchet wrench into the extension, then lefty-loosey. This might take some effort to break loose, but once you've done that, you can just pull the tool off and unscrew the sensor the rest of the way by hand.

You should be rewarded with an ugly old oxygen sensor. Here's a shot of the new and the old. The new one is on the left, the old one is on the right:



Installation is the reverse of removal. The oxygen sensor should be torqued to 50Nm or 37ft-lbs. If you purchased your sensor from BMW, then it's probably already coated with a little anti-seize compound. If not, then consider putting some on. I recommend screwing the sensor in by hand as much as possible to avoid cross-threading, which could really be costly here.

Note that when plugging the other end of the wire into the connector, it is unidirectional, so it's impossible to jam it in the wrong way. See how the connector has unique tabs on either side:



Slide it into the connector, snap the clips on, and clip the boot back on. Put the connector and the wire back in the clips they came out of. Then replace everything else as the reverse of above.

I hope this helps somebody out there. I read lots of stories about problems with cold idling; this could be the answer for some of you!
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Old 11-26-2005, 08:28 PM   #4
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Ahh, that removal of the cabin air filter would have made change of spark plugs a TON easier for the inner two plugs.
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Old 11-27-2005, 10:45 AM   #5
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Awesome did you notice an improvement in gas milage?
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Old 11-28-2005, 08:36 PM   #6
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Quote:
Originally Posted by joserse46
Awesome did you notice an improvement in gas milage?
Thanks. Much too early to tell, but I'll post back if it's significant. I also put my snows on this week with the 16" rims, so that will probably bump up my fuel economy too. Just another variable in a poorly controlled experiment!
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Old 11-28-2005, 09:38 PM   #7
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Great write-up. New brakes, O2 sensor, fluids....no stopping you now
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Old 11-29-2005, 01:40 AM   #8
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o2

best write up on o2 I've seen........great pics
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Old 12-19-2005, 02:23 PM   #9
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Such a great, detail oriented DIY. And it couldn't come at a better time too...

Thanks.

This brings up a quick question. And I'm in no way knocking down your work. You obviously do a great job with a great deal of attention to details. But (aside from O2 Sensors being a pretty pricey if you're changing all 4) yet, wouldn't you want to change 'em all at the same time?

Again, an AWESOME DIY.

S
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Old 11-29-2005, 09:44 AM   #10
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Thank you very much!!! Now I am no longer afraid to try this on my own.
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Old 11-29-2005, 02:17 PM   #11
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Excellent! Well thought out and well documented. Anyone who needs to ask a question after reading your post should not attempt any repairs. (J/K)
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Old 11-29-2005, 07:08 PM   #12
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Excellent, nicely done DIY.... I'm sure you'll help someone with it down the Rd
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Old 11-30-2005, 11:48 AM   #13
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Great write-up, thanks for taking the time to do this.
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Old 12-14-2005, 07:38 PM   #14
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I changed mine over the weekend. From my experience, I couldn't use the socket type because the OS was too tall. I had to use a crows feet type.

When I tried placing things back, I accidently damage that thin vacuum line running along with the OS wiring for the air pump. Being brittle over the years, I bumped it and it bended and cracked. Loom the wiring over the vacuum line next time. FYI
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Old 03-23-2006, 10:09 PM   #15
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Great job Rox!
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Old 07-18-2006, 08:02 PM   #16
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is there anywhere else to get 02 sensors cheaper?? bavauto.com sells them for 100 for pre-cat and 110 for post cat each. i need to get mine replaced as well since they are throwing the SES light.. codes p1188 n p1189..

after reading.. it seems that usually the pre-cat sensors are the ones that need to be replaced rather than the post cat.. does p1188 and p1189 mean that both pre and post sensors need to be replaced ?? why are there two codes? thanxx
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Old 07-18-2006, 10:08 PM   #17
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Before you go and spend the money on the O2 sensors, make sure that's actually your problem. Although all my errors indicated O2 sensors, the root cause turned out to be a defective crankcase vent valve (aka oil separator). My shop wanted to charge me $1500 to change all 4 O2 sensors. Luckily I went for a second (and third, and fourth) opinion, and decided to try replacing the $50 part first (although the labour to do so turned out to be a nightmare). Once that part was replaced and the codes reset, car has been fine. Telltale signs are a fog horn sound when you turn your engine off.

Quote:
Originally Posted by 6strings
is there anywhere else to get 02 sensors cheaper?? bavauto.com sells them for 100 for pre-cat and 110 for post cat each. i need to get mine replaced as well since they are throwing the SES light.. codes p1188 n p1189..

after reading.. it seems that usually the pre-cat sensors are the ones that need to be replaced rather than the post cat.. does p1188 and p1189 mean that both pre and post sensors need to be replaced ?? why are there two codes? thanxx
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Old 07-18-2006, 11:11 PM   #18
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Great, clear DIY. Nice job on the writeup.
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Old 07-19-2006, 07:23 AM   #19
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Quote:
Originally Posted by FFWD
Before you go and spend the money on the O2 sensors, make sure that's actually your problem. Although all my errors indicated O2 sensors, the root cause turned out to be a defective crankcase vent valve (aka oil separator). My shop wanted to charge me $1500 to change all 4 O2 sensors. Luckily I went for a second (and third, and fourth) opinion, and decided to try replacing the $50 part first (although the labour to do so turned out to be a nightmare). Once that part was replaced and the codes reset, car has been fine. Telltale signs are a fog horn sound when you turn your engine off.
there is a fog horn sound but its not a loud sound.. when i turn my engine off i have to open the door in order to hear that sound. it sounds like the fan turns off and then theres this faint little fog horn sound like "errrrr" for a few seconds.

where can i find this 50 dollar part? is it a diy? all the signs point to o2 sensors tho.. slower performance, delayed acceleration.. did u still get these signs even with that defective crank case valve?? thanx for ur input!
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Old 07-22-2006, 10:31 AM   #20
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Yeah I wanna Know where to get the $50 dollar part and the DIY on it.
I replaced the Pre-cat O2 Sensors but the light is still on and The car won't start after its been on for awhile. Or what else could the problem be?
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