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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 05-11-2014, 04:57 PM   #1
Registered User
Join Date: May 2014
Location: Buckinghamshire
Posts: 2
My Ride: M3
How to replace pre-cat Lambda (O2) sensor

I have just replaced a faulty O2 Lambda sensor on my E46 M3 so thought I'd post a few pics to help others. I bought Bosch sensors from eBay for around 58 each after the fuse blew on a track day and the car went into limp mode.

The diagnostics said Sensor 1, Bank 1 heater failure, which I interpreted as the internal heater element in the sensor was shorting and blowing the fuse.

You will need:

- Metric socket set with plenty of extensions (10", 12") and universal joint adapter to get into angles
- 10mm and 13mm spanners
- hex bolt keys
- 22mm crows foot socket
- a bucket of patience

Steps I followed:

- Jack car up safely. Use axle stands

- Remove plastic engine under-tray (seven screws) so you can find all the bolts you drop through the engine bay. Leave the tray on the floor under the car to catch your mistakes

- open bonnet and remove the following:

Pollen filter housing and tray. Let the cable harness than runs in front of the tray just dangle free.

Torsion bar (two bolts on either side).

Right side (drivers side in UK car) engine fan shield that curves around the engine block and is secured with two plastic pin clips.

Remove oil filler cap and breather pipe from top of injection cover (squeeze and pull), then take the whole cover off itself (6 acorn bolts). Put the oil filler cap back on.

Remove the EGR valve thing from the side of the engine block. It has a pipe that goes into a vacuum pump.I suggest you disconnect the pipe at the pump end only. Just squeeze the sides of the connector and pull.

Look along the side of the engine block. There is a heat shield covering the exhaust manifold. We need to get under there. Removing all the bolts holding the shield on is a bit fiddly. A spanner was the easiest.

Once the shield bolts are off you need to slide it towards the front of the car to gain access to the lambda sensor below. I found I had to undo the water reservoir pipe and the bolt holding the metal water pipe that disappears around the front of the block. Once the shield was back enough I reconnected the reservoir pipe.

It also helped to disconnect one of the electrical connectors at the bulkhead end of the block.

Once you've wiggled the heat shield back, you can see the pair of O2 sensors (bank 1, sensors 1 and 2) sticking out of the exhaust pipes where the three sections of pipe merge into one (cylinders 1-3 and cylinders 4-6).

Now unplug the sensor from the socket held on top of the injector block. Use a small screwdriver to unclip each side, then pull apart.

Free the sensor lead from the clips within the injector block and around the side of the heat shield so it is totally free all the way down to the sensor. There is a clip around the back of the block against the bulkhead also. Be careful not to pull the clip out.

I used a crows foot socket on the end of a long extension bar with an angled knuckle at the end.

I sprayed plenty of penetrating fluid on the sensor a few hours beforehand but ended up having to run the engine for a few minutes to heat the manifold up before the sensor would finally come loose. This meant reconnecting the EGR and water reservoir pipes before starting the engine.

To insert the new sensor I dropped I got my wife to dangle the sensor on the wire from the top whilst I reached underneath the car to feel the sensor into the hole. We found if she twisted the cable clockwise it helped me turn the thread in a few turns. Then I used the crows foot to finish the job.

Putting everything back is a reverse procedure.

if a fuse blew due to the sensor heater failure, replace the green 30Amp fuse in the fusebox here.

Once done, I checked the sensor was working properly by looking at the voltage levels on the O2 sensor using Torque on my android phone connected to the ECU via a 10 bluetooth OBD adapter. The sensor in question is the bottom left dial and graph (O2 1x1).

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