Audi A6 C5: How to Replace Oxygen Sensors
Replacing old oxygen sensors can help improve the fuel efficiency in your Audi A6. This article will show you how to do it.
This article applies to the Audi A6 C5 V6 (1998-2004).
Oxygen sensors are important for measuring the output of your engine. They can help your car's computer to calculate the air/fuel mixture in the engine and improve its fuel efficiency. But after many miles of wear and tear, these sensors can get corroded and stop working. On most vehicles an oxygen sensor would not be so difficult, but on the Audi A6 it's a little more challenging. But that doesn't mean you need to pay the extensive labor costs of a dealership just to change an oxygen sensor. Check out the instructions below to learn more.
Materials Needed
- Replacement O2 sensors
- Ratchet
- 22mm O2 sensor socket
- 2 feet of ratchet extensions
- Universal socket
- 10mm socket
- Breaker bar
- Phillips screwdriver
- Flat head screwdriver
- PB Blaster
- Anti-seize
- Needle nose pliers
- Zip ties
- Duct tape
- Jack and jack stands
- Wire cutter
- Crimp connectors and wire crimper
- Heat shrink or electrical tape
Since the pre-cat sensors are so much more difficult to replace, we'll be focusing on those. The process for the post-cat sensors are similar, but you can skip the first two steps.
Step 1 – Remove engine cover, air filter cover, and coolant reservoir
Start by removing the air filter cover. The cover just lifts off. Next, remove the engine cover. There are a series of six screw taps that need be removed first, and the cover pieces can then be lifted off.
Removing the coolant reservoir is only necessary if you're replacing the driver's side pre-cat sensor. There are three Phillips screws along each outside corner of the reservoir. Unplug the sensor for the coolant reservoir. Then move the reservoir out of the way.
Figure 1. Remove the air filter and engine covers. 
Figure 2. Remove the coolant reservoir and set it aside.
Step 2 – Remove MAF sensor and large air hose
This is only necessary if you are removing the passenger's side pre-cat sensor. Unclip the fuel lines from the air filter housing and move them out of the way. Disconnect the MAF sensor housing from the air filter housing by undoing the two clamps on opposite sides of the housing. Then set the MAF housing aside. Now remove the large air hose connected to the back of the engine. This can be removed by squeezing the clamp to release it.
Figure 3. Unclamp and remove the MAF sensor housing. 
Figure 4. Remove the large air hose.
Step 3 – Remove the O2 sensor
Start by disconnecting the electrical harness for the respective O2 sensor. All of the electrical connectors are lined up along the fire wall in the rear of the engine compartment. All but one can be found near the air filter. The driver's side pre-cat sensor harness will be found closer to the sensor itself behind the engine.
Figure 5. Disconnect the O2 sensor wiring harness. 
Figure 6. The driver's side pre-cat O2 sensor connector is separate from the other connectors.
The two pre-cat sensors are the hardest to locate. The driver's side pre-cat O2 sensor can be seen behind the driver's side of the engine once the coolant reservoir is removed. The passenger's side pre-cat O2 sensor is much harder to pinpoint, but it's between where the large air hose sat and the engine. If it's too hard to find, try following the exhaust back to the sensor.
By now you should be able to get an angle on the O2 sensor. Assemble your ratchet with the necessary sockets and extensions. Then slip the O2 sensor socket over the O2 sensor. Loosen the sensor from the exhaust. If your sensors are really old, they may be difficult to remove. In this case use a breaker bar to crack it loose, then switch back to a regular ratchet. In extreme cases you may want to douse the socket in PB Blaster or a similar lubricant and wait 15 minutes before attempting to loosen it again.
Figure 7. Driver's side pre-cat O2 sensor. 
Figure 8. Passenger's side pre-cat O2 sensor.
Pro Tip
Use tape to hold together your ratchet components so they don't fall into the engine bay.
Step 4 – Install new O2 sensor
There are two kinds of O2 sensors you can use to replace your old sensors. You can get a sensor that's exactly like the old sensor with the same type of connector, or you can get a universal sensor with no connector. You may find that it's easier to get a universal sensor and reuse the old sensor's electrical harness rather than trying to remove the harness.
If you are installing a universal sensor, then you'll need to cut the old sensor from its electrical harness. Then use crimp connectors to connect the new sensor wires to the old harness. Then you must wrap the connections in heat shrink or electrical tape to protect the wires from environmental damage. Finally you just install the sensor like normal. If you're replacing the old sensor harness, then you'll have to cut the ties that hold the harness and hang up the new harness with zip ties.
When installing the new sensor, you should thread it in by hand so you don't accidentally damage the wiring harness. Then, once it's tight, give it another turn or two with the ratchet. Once the sensor is installed to the exhaust you can plug in the electrical harness. Then you're all done. All that's needed is to put back anything you removed in order to access the sensors, such as the coolant reservoir, air hose and MAF sensor.
Pro Tip
If you have a fault code from your O2 sensor, you can clear the code using VAG-COM software and a scan tool.
Featured Videos: Audi C5 S6 Oxygen Sensor Replacement
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Driver's Side
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Passenger's Side
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