The catalytic converter on your Audi A4 is a crucial part of the car's emissions system. It is situated between the exhaust header and the end of the exhaust that exits the car. Its job is to convert environmentally harmful exhaust gasses into less harmful gasses. But catalytic converters also rob the motor of power, which is why race cars do not use them. You can bypass a catalytic converter with a test pipe available from companies that sell Audi performance parts. Keep in mind, though, that in most states this is illegal and should only be done on cars that are not driven and registered on the street.
Loosen the lug nuts on the wheels. Jack up the Audi and place it securely on jack stands at all four corners.
Remove the front oxygen sensor, located near the exhaust down pipe. This is accessed from the engine bay.
Using a 17 mm socket, remove the top nuts that connect the catalytic converter to the turbo. In a nonturbo car, the exhaust will simply connect to the catalytic converter.
Unhook the red wire connector for the second oxygen sensor. The connector is on the fire wall on the driver's side. Follow the wire to oxygen sensor, and disconnect it from the sensor.
Remove the bottom 17 mm bolts that connect the catalytic converter to the turbo. Remove the spring-loaded bolt that secures the down pipe.
Remove the cross member that goes across the exhaust, using a 10 mm 12-point Torx bit. Loosen the nuts that hold the exhaust collar. Slide the collar toward the back of your car.
Slide the catalytic converter down and out along with the down pipe. This will take some twisting and turning to release it, but it should come out fairly easily.
Remove the catalytic converter from the down pipe. These are held together with three 12 mm nuts and three 13 mm bolts.
Install the test pipe in place of the catalytic converter. Reinstall the down pipe with the test pipe in reverse order of removal, bolting it back to the turbo on the top and bottom.
Reinstall the cross member, exhaust collar and oxygen sensors in reverse order of removal.
