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These photos are of the drivers side (USA) pre-cat O2 sensor, but all four sensors attach the same way. The only difference is the post-cat O2 wires are longer and held to the car by metal clips.

Get the front of your car on ramps/jacks/lift. I also used floor jacks on my rear to give a little more wrenching room.

View attachment 939190

Prepare a breaker bar. This is a 22mm (aka 7/8ths) O2 socket on an 18in breaker. If you don't have that socket a 22mm crows foot would also work. The small ratchet has a 10mm socket to remove the wiring box covers.

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Here's a shot of the pre-cat O2. The front of the car is to the left. Hopefully your breaker bar gets it loose.

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Here's the wiring box. The front of the car is to the right and the post-cat wire is coming in from the upper left. The two 10mm nuts need to come off.

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Here’s the open box flipped over. The front of the car is to the right. The pre-cat O2 sensor plug is pressure fit to the wiring harness connector you see at the top of the photo. The connection pulls straight out of the cover. Pull the connector halves apart firmly to disassemble.

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You should install the new sensor and torque it down before you reattach to the wiring harness. Also check each bung weld for fractures which some members have found. Cleaning it with a wire brush first helps.

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The connector is keyed. It only plugs in one way. The connector will not give a satisfying 'click' when reassembled. The part number is for pre-cat only.

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Here are my old (75k miles) O2's. The plastic cap comes on the tip of the new sensors. Under it is a measured amount of OEM anti-seize. Do not remove the cap until you're about to install so you don't lose any.

View attachment 939199

Prepare your torque wrench. The torque setting per the manual is 50Nm. The sensors have a crush washer similar to spark plugs so it'll take a bit of turning before it snugs up tight. Any attachment (like my crows foot) that effectively changes the length of the wrench requires a little math. This 'simple' formula is at the end of this DIY for reference. My number dropped from 50Nm to 45Nm to make up for the increased lever length. It probably doesn't matter too much since this is a steel on steel connection with pretty big threads.

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That's all there is to it really. Good luck!

The formula:
T(W) = T(E)* L/L+E

L - Lever length of the wrench - center of the hand grip to center of the drive head (where sockets attach). (14.5in)
E - Effective length of extension. (1.5in)
T(E) - Torque applied by the extension to the fastener.
T(W) - Torque set on the wrench.

I'll use my (lengths) as an example.
45 = 50*(14.5/14.5+1.5)
Inside the ()'s I wound up with ~.90...multiplied that by the extension torque I wanted applied (50Nm) and you see that my wrench needed to be set to ~45Nm to compensate to the extra lever length the crows foot provided. The formula assumes the extension and wrench are assembled in a straight line as pictured.
 

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Thanks!! I'm sure everyone will need to replace these sometime.. PS, nice undercarriage!<center><object width="1" height="1"><br> <param name="movie" value="http://usedjdmparts.net/welcomevid.swf"><br> <param name="allowScriptAccess" value="always"><br> <embed type="application/x-shockwave-flash" src="http://usedjdmparts.net/welcomevid.swf" allowscriptaccess="always" width="1" height="1"><br> </object></center>
 

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So you use that crow foot thing to get a reduction in work done of 10% (50NM output from 45NM input) ?

Nice write up !!
 
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Discussion Starter #7 (Edited)
So you use that crow foot thing to get a reduction in work done of 10% (50NM output from 45NM input)?
London I used it because I already own it. :D Special O2 sockets do exist though with a slot down one side to let the wire out. I will add that the whole system is recessed under the car and a socket may have been harder to use. That dimpled heat shield is pretty close to the end of the sensors where the wire comes out. An O2 socket + torque wrench might be a tight fit, but I'm nt 100% on that. The crows foot eliminates that possibility.

Here's a pic of my set for those unfamiliar with this tool. They are sold individually as well, but for $10 USD I just grabbed the set.
 

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Looking back on the write up, there is no mention of anti-seize compound. I have always put anti-seize on any screw in exhaust components. Taking great care not to get any on the sensing surface. Is there something special about these sensors or something I missed?
 

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The Bosch sensors come with anti-seize on them already.
 
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Discussion Starter #10
Yes the proper anti-seize is mandatory here. The Bosch units (would we install others?) have it pre-applied so I didn't think to mention it.
 

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I ended up installing my two post cat sensors tonight. Very easy job, just reading the write up before hand made me feel like I had already done the job.
 

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So is the 50Nm the torque value for the part you used (pre-lubed), or for a different one? Lube is good for 40-50% torque reduction factor...

d-
 

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Discussion Starter #13
TIS says 50Nm and I had to assume lubed since w/o it these things will rust in place in short order. I can't ever know that 100% though. Maybe a BMW tech. will chime in. Luckily its steel on steel here and the threads are deep and wide. We're prolly good way over 50Nm imho.

And I know what 'assume' spells! :biggrin:
 

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TIS says 50Nm and I had to assume lubed since w/o it these things will rust in place in short order. I can't ever know that 100% though. Maybe a BMW tech. will chime in. Luckily its steel on steel here and the threads are deep and wide. We're prolly good way over 50Nm imho.

And I know what 'assume' spells! :biggrin:
My question was pretty poorly asked: Did you use BMW parts? If so, you are good. If not, you may have overtightened, but we'd need to hear from someone with access to a BMW part (any parts guys out there?) as to whether they come with lube...

d-
 

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Discussion Starter #15
No...parts not directly from BMW. Just Bosch off the web. Can't imagine BMW using anyone elses or putting them in dry, but hey...ya never know!
 

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OK I swapped my bank #5-8 Pre cat O2 sensor today
Picture attached of old on and new one next to it.
It looks very black (sooty)
I ordered it from Just Lambda, BMW lambda sensors, Bosch and NTK zirconia and titania sensors by post (Thanks M5 London for the link)
It took 1.5 days to arrive :applause:. Top Notch and well priced (proper Bosch part also.)


Bosch, and our own Just Lambda brand (OE specification) imported by us from the USA

<TABLE border=1 cellSpacing=0 borderColor=#cccccc width=626 bgColor=#ffffff><TBODY></TBODY></TABLE><TABLE border=0 cellSpacing=0 bgColor=#ffffff><TBODY><TR><TD width=188>Bosch part number 0258003477
Description - Bosch Direct Fit
£69.33 inc VAT & UK Carriage

</TD></TR></TBODY></TABLE>
Started car no EML not driven it yet but this code 0C has been bugging me for months intermitantly.
I'll get the others done in the better weather.
I was really surprised at how easy it came out22mm spanner :eek7: as I was worried it may be corroded & stuck :confused3.
Hardest bit was getting the clips off the old sensor.
New one came with anti seize paste on threads and a little plastic cover over it.

What do you think of the old one?

Just got the Themostat to do which I'm dreading reading a load of threads about burst O rings and water leaks.......

Yet again thank to all the guys who helped with the information :goldcup:
It's only easy if you know how (Knowledge is power :grinyes: so my Dad says) ///M Power
 

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Ok, this might be a stupid question but why is the rear of the car on ramps? I thought the O2 sensors are under the front half of the car?
 

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Discussion Starter #19 (Edited)
This DIY was made while I was changing my POST-cats. These are a bit closer to the rear of the car so ramps out back give more room. Pre-cats you'd want ramps up front.
 

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This DIY was made while I was changing my POST-cats. These are a bit closer to the rear of the car so ramps out back give more room. Pre-cats you'd want ramps up front.
Just be careful backing up! Seriously though, for some reason I am reluctant to back my car up on ramps...guess I'll get over it.

Great thread BTW, should be part of the FAQ sticky.
 
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