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Please read through all of the instructions here and make sure you understand them before attempting this procedure!

Oxygen sensors screw in to the exhaust manifolds (one in each manifold) and tell the ECU if the engine is running rich (too much fuel) or lean (not enough fuel). Depending on your driving habits, the sensors will last anywhere from 30,000 miles (48000km) to 75,000 miles (121000km) or more. If your fuel economy has recently taken a bit of a dive, and your engine idle has become a bit rough, you might need oxygen sensors. For a more scientific test, you can connect the positive lead of a voltmeter (an old-fashioned analog one works best) to the black wire that comes out of the sensor, and the negative lead of the meter to a good ground. With the car at idle, the voltage coming off the sensor should vary from a little over 0v to a little under 1v every few seconds. If the voltage range is less than about .6v, the sensor needs to go.
As far as Audi repairs go, replacing the oxygen sensors is essentially child's play. If you have genuine Audi sensors with genuine Audi plugs on them, the procedure takes mere minutes. If you have reasonably priced sensors with non-Audi plugs on them, it can still take less than an hour. Go for it!

Tools Required:

  • Torque wrench
  • Oxygen sensor socket
  • Flex adapter for wrench
  • One or two 8"-10" wrench extenders
  • Wire cutters/strippers
  • Soldering iron, solder, flux, and heat shrink
    -or-
  • Crimpers, butt splices, and electrical tape


Parts Required:

  • 2 Oxygen sensors , Bosch P/N 13913. Get two, even if you think you only need one. You need two. Also, beware of generic or store brand replacements! They are often inferior and will cause you lots of grief. If someone runs up to you in the middle of the night, and tries to give you two non-Bosch oxygen sensors, JUST SAY NO. The sensors you want are Bosch 3-wire heated oxygen sensors. All Bosch 3-wire heated oxygen sensors are identical except for the plugs on the end, which you're cutting off anyway, so if your store doesn't have Bosch 13913 sensors, get different Bosch sensors, as long as they're BOSCH 3-WIRE HEATED OXYGEN SENSORS.
  • If your car has ODB-II (All A4's, All A6 1996 and later, All Cabriolet 1996 and later) you need 4 oxygen sensors, not two.


Procedure:

Start this procedure with a cold engine -- you'll be working around the exhaust manifolds, which get very hot when the engine runs. I'm going to write instructions to change the sensor on one side of the engine -- both sides use the same procedure, so you can do them both at the same time or one right after the other, but I'm only going to write the procedure once.
If you have an ODB-II car (A4, A6 1996->, Cabriolet 1996->), click here for extra steps.

  1. Find the sensor on the exhaust manifold, and trace its wires to where it plugs into the car's wiring harness. The passenger's side connector is behind a row of connectors in the middle of the firewall -- don't worry about disconnecting the other connectors. They're all color coded for easy re-connecting.
  2. Disconnect the oxygen sensor wires. There are two connectors: one with a single black wire (the signal) and another with two white wires (the heater). Untangle the wires and set them somewhere where they won't get tangled again.
  3. Get your O2 sensor socket and slip it over the sensor. Feed the wires through the slit in the side of the socket, and push the socket down until it completely engages the hex part of the sensor.
  4. Attach your flex joint to the sensor socket, and then attach your extension(s) to the joint. Remember that the joint doesn't work miracles, so don't try any really funny angles -- try to make the whole thing as straight as possible.
  5. Twist! Sometimes the sensors pop right out; other times they are really stuck and take a lot of huffing and puffing before they come out. If it's too hard, spray some penetrating oil around the threads of the sensor and let it sink in a bit before trying again.
  6. After you have the sensor out, cut the three wires off as close to the sensor as you can -- you want to save the wires and the connector(s), but not the sensor. You can now pitch the old sensor.
  7. Remove the new sensor from its box. You'll notice a plastic cap around the tip of the sensor -- don't remove it!
  8. Cut the three wires off as close to the plug(s) as you can -- you want to save the wires and the sensor, but not the plug(s). You can now pitch the new plug(s).
  9. You now have one good sensor and one or two good plugs, and you need to connect them. The two white wires are interchangeable; it does not matter which white wire from the sensor goes to which white wire on the plug. For the purposes of this tutorial, I'll use the Behold Method (Behold, the wires are connected!). If you aren't comfortable with that, read the soldering guide, the crimping guide, and the do's and dont's of splicing, and decide for yourself which method to use. Personally, I don't trust butt splices farther than I could comfortably spit a rat, but that's a matter of great debate (I mean the trustworthiness of butt splices, not my rat-spitting abilities). Use whichever method makes you more comfortable.
  10. Take the completed sensor back to your car, and carefully remove the plastic cap (the cap was still on, right? Good.) and carefully thread the sensor, by hand, into the exhaust manifold. Remember that the sensor sticks out at a funny angle, so if the threads aren't catching, the sensor is not lined up properly. It should thread in easily, so if it isn't going in easily you're doing it wrong. Once the sensor is in hand tight, whip out your oxygen sensor socket and tighten the sensor to 50Nm (37 ft lbs). If you aren't using a torque wrench, that's very tight -- tighten them until you feel the copper washer start to compress.
  11. Connect the wires to their proper plugs. One nice thing that you'll notice is that in addition to most connectors being color coded, if a connector needs to plug in a certain way, that's the only way to plug it in. Which is to say, it's not possible to plug in a connector backwards, unless you break it. Which means that if it's hard to plug in the connector, try plugging it in a different way.
  12. Done! At this point, some people like to disconnect the ground strap of the battery to reset the ECU's oxygen sensor learning function. If your car was manufactured in 1992, don't bother because your ECU does not have this function. If you want to disconnect the ground strap, make sure that you have things like the radio code, because you'll need them when you re-connect the battery. Disconnect the GROUND strap and leave it off for at least 1 minute. Be careful as you re-connect it -- slip it straight on the battery post; don't let it bounce. You may see a few small sparks as it goes back on; don't let them bother you too much. The less you move the wire on the post, the fewer sparks you'll see. Don't touch anything metal while you're doing all of this. If you don't want to mess with the battery, it's OK -- the ECU will learn your new sensors. It takes a bit of time, and the check engine light might come on while it's learning, but it should work itself out in a few days.
  13. Woohoo! Now you're really done. Take a test drive; remember that the ECU needs to learn your new sensors, so you might not feel an amazing difference right away.



I've tried to make this procedure as complete as possible. However, if you notice any ommissions, or something I've gotten wrong, or have good quality pictures of your engine while you're in the midst of this procedure, please send me an e-mail.


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