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-   -   Pre-cat Oxygen Sensor DIY (lots o pix) (https://forum.e46fanatics.com/showthread.php?t=313882)

r0x 11-26-2005 03:40 PM

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:


r0x 11-26-2005 03:42 PM

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.


r0x 11-26-2005 03:43 PM

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!

bluskye 11-26-2005 08:28 PM

Ahh, that removal of the cabin air filter would have made change of spark plugs a TON easier for the inner two plugs.

joserse46 11-27-2005 10:45 AM

:thumbup: Awesome did you notice an improvement in gas milage?

r0x 11-28-2005 08:36 PM


Originally Posted by joserse46
:thumbup: 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! :P

backdoc 11-28-2005 09:38 PM

Great write-up. New brakes, O2 sensor, fluids....no stopping you now :thumbup:

trickydik_1 11-29-2005 01:40 AM

best write up on o2 I've seen........great pics

Steel Blue 11-29-2005 09:44 AM

Thank you very much!!! Now I am no longer afraid to try this on my own.

rayxi 11-29-2005 02:17 PM

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) :)

Surk 11-29-2005 07:08 PM

Excellent, nicely done DIY.... I'm sure you'll help someone with it down the Rd :thumbup:

Alex 11-30-2005 11:48 AM

Great write-up, thanks for taking the time to do this. :thumbup:

K:^P 12-14-2005 07:38 PM

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

Speeder 12-19-2005 02:23 PM

Such a great, detail oriented DIY. And it couldn't come at a better time too...

Thanks. :thumbup: :clap:

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.


FFWD 03-23-2006 02:12 PM

Original or aftermarket sensors?
Does anyone know if aftermarket/OEM sensors are available, and if so where I could buy some? Apparently my car needs all four replaced! :yikes:
Or if not, where is the best place to buy original equipment via mail order?


FFWD 03-23-2006 02:17 PM

To answer my own question, I just realized bavaut.com has them. Can anyone attest to the quality of these ones? I'd still be interested in alternate sources.

K:^P 03-23-2006 07:06 PM

i think 13802 is a bosch number. It should be fine. Regarding changing the two after cat OS, it may not be necessary. If the computer doesn't kick back an error code, I'm not spending extra money to change it. Besides, to change the after cat OS, it doesn't look easy.

buddy325i 03-23-2006 10:09 PM

Great job Rox!

FFWD 03-23-2006 11:52 PM

According to my shop, only the two pre-cat sensors are shorting. They said I should change all four since the two post-cats will probably go within a few months. When I asked about changing only the two that are faulty, they said it was like changing only a few of your spark plugs, because all four work in unison, and if I put in new pre-cat sensors, the readings to the computer might be incorrect because the new sensors would be more accurate than the old sensors. Does this make any sense? I don't mind spending the money to fix a problem properly, but I hate to replace a part that doesn't really need replacing (yet). Also, anyone have any opinions on the bavauto.com sensors?


Originally Posted by K:^P
i think 13802 is a bosch number. It should be fine. Regarding changing the two after cat OS, it may not be necessary. If the computer doesn't kick back an error code, I'm not spending extra money to change it. Besides, to change the after cat OS, it doesn't look easy.

Tom Busby 03-24-2006 08:59 AM

The post above, where the mechanic advised changing all four oxygen sensors, is good adivice from a shop because they (and their customers) do not want complaints and returns. However, if you do all your own work then letting the back pair of sensors stay in place until they fail is a good strategy because you can't not-notice a failed sensor, and the back pair have different operating conditions than the front pair.

Awesome write-up and pictures on this thread by rOx.

And prescient screenname-choice by rOx don't you think?

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