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Old 12-24-2010, 11:11 PM   #1
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Join Date: Dec 2010
Location: MD, USA
Posts: 1,489
My Ride: 01 330CI 6MT ZSP EU3
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 11:14 PM.
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