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Audi A4 Catalytic Converter Replacement
 
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Pelican Technical Article:

Audi A4 Catalytic Converter Replacement

Steve Vernon

Time:

4 hours4 hrs

Tab:

$650

Talent:

***

Tools:

22mm, 17mm, 13mm wrench, penetrating oil, floor jack, jack stands, wheel chocks, safety glasses, torque wrench, auto creeper

Applicable Models:

Audi A4 (2002-05)
Audi A4 Cabriolet (2003-06)

Parts Required:

New catalytic converter, gasket, connecter

Hot Tip:

Don't work on a hot engine

Performance Gain:

Pass a SMOG test

Complementary Modification:

New O2 sensors

There are millions of cars on the road, and each one is a source of air pollution. The job of the catalytic converter is to convert harmful pollutants into less harmful emissions before they ever leave the car's exhaust system.

In order to reduce emissions, modern car engines carefully control the amount of fuel the engines burn. Engines try to keep the air-to-fuel ratio very close to the stoichiometric point, which is the ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For an internal combustion engine running gasoline as a fuel source the stoichiometric ratio is about 14.7:1, meaning that for each pound of gasoline, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be lean (an air-to-fuel ratio higher than 14.7), and other times the mixture can be rich (an air-to-fuel ratio lower than 14.7).

Most modern cars are equipped with three-way catalytic converters. This refers to the three regulated emissions it helps to reduce.

The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2.

The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas.

The third stage of conversion is a control system that monitors the exhaust stream, and uses this information to control the fuel injection. There is an O-2 sensor mounted upstream of the catalytic converter, meaning it is closer to the engine than the converter. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is running at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO.

If your car cannot pass a SMOG test and the catalytic converter is found to be faulty, this article will take you through the steps to replace it.

Let the car cool so you don't have to work around a hot engine. You are going to need to safely lift and support the vehicle as high in the air as safely possible. Please see our articles on safely raising and supporting your vehicle for additional assistance.

The catalytic converter is located on the right side of the motor; it is attached to the turbo and has two O-2 sensors attached to it.
Figure 1

The catalytic converter is located on the right side of the motor; it is attached to the turbo and has two O-2 sensors attached to it. It is a tight fit accessing the exhaust flange to remove the bolts but you should be able to reach everything once you remove the engine cover. This photo illustrates the room you will have to work on the catalytic convertor where it is mounted to the turbo (yellow arrow) and the two O2-sensors (red arrow, one shown).

The best place to begin is by using a 22mm wrench or the 22mm craw-foot socket and remove the O-2 sensor from the top of the catalytic converter (red arrow).
Figure 2

The best place to begin is by using a 22mm wrench or the 22mm craw-foot socket and remove the O-2 sensor from the top of the catalytic converter (red arrow). Note: you may need to soak the plug with some penetrating oil to help remove it.

Use your wrench or socket and remove the second O-2 sensor (red arrow, not shown).
Figure 3

Use your wrench or socket and remove the second O-2 sensor (red arrow, not shown).

Use a 17mm socket and remove the three nuts holding the cat (catalytic converter) pipe to the turbo (red arrows, two shown).
Figure 4

Use a 17mm socket and remove the three nuts holding the cat (catalytic converter) pipe to the turbo (red arrows, two shown). Note: you may need to soak the nuts with some penetrating oil to help remove it.

Working below the vehicle, remove the two 17mm nuts holding the connecter to the cat pipe and exhaust pipe (red arrows).
Figure 5

Working below the vehicle, remove the two 17mm nuts holding the connecter to the cat pipe and exhaust pipe (red arrows).

Move forward to where the cat pipe comes down from the turbo.
Figure 6

Move forward to where the cat pipe comes down from the turbo. There is a bracket welded to the pipe that connects to a bracket on the vehicle (red arrow).

There is a 13mm nut and bolt that passes through a spring (red arrow) to eliminate vibrations.
Figure 7

There is a 13mm nut and bolt that passes through a spring (red arrow) to eliminate vibrations. Use a 13mm wrench and remove the nut and bolt. Use care, since once you remove this bracket the cat and pipe will be free. It is a tight fit removing it from the vehicle. You will have to twist and turn the pipe to get it both out and in but take your time and remove the old one and install the new one.

It is a good idea to use a new connecter when attaching the pipe from the Cat to the exhaust.
Figure 8

It is a good idea to use a new connecter when attaching the pipe from the Cat to the exhaust. The connector has an inner sleeve that compresses around the pipes when tightening (red arrow). The old sleeve may not open enough or be too corroded to reuse so do yourself a favor and order a new connector when ordering the cat.

When installing the new connector make sure that when you mount it the nuts and bolts are NOT positioned below the lowest height of the connector (red line and yellow arrow).
Figure 9

When installing the new connector make sure that when you mount it the nuts and bolts are NOT positioned below the lowest height of the connector (red line and yellow arrow). Installation is the reverse of removal. Do NOT forget to install a new gasket where the Cat meets the turbo.

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Comments and Suggestions:
chai Comments: Does acceleration hesitation in the car is caused by catalytic converter? I have an Audi A4 2005 1.8T and the car jerks and hesitates to accelerate on RPM's above 2000 with CEL blinking. Could you please advice?
December 1, 2016
droth Comments: I am thinking of doing this, but in reference to the question by luchomame July 12 2016 If using jackstands will there be enough clearance to pull the cat down and out or would one need to be on a lift to get more ground clearance
September 19, 2016
  Followup from the Pelican Staff: You should have enough room. Get the vehicle up on four jack stands to be sure. Lift all four wheels. - Nick at Pelican Parts  
Marius Comments: Do you know what year, if it was Quattro, and what transmission this Audi was? Depending on if you have a Quattro or FWD, manual or automatic, the procedure is different..... thank you!
August 17, 2016
  Followup from the Pelican Staff: 2003 Audi A4, auto trans, FWD, (non-quattro) - Nick at Pelican Parts  
luchomame Comments: so i have done all of this and i cant get the cat out. any tips? i am using jackstands.
July 12, 2016
  Followup from the Pelican Staff: If the instructions were followed it should be ready to come out. If not, check where it is binding. Maybe it is corroded. - Nick at Pelican Parts  

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