V6 Catalytic Converter And O2 Sensor Replacement

by: ashman78


The following writeup covers the replacement of the catalytic converter and both oxygen sensors (per side) in early B5 V6 engines, most commonly found in the VW Passat, Audi A4, and Audi A6 FWD models. AWD models (quattro and 4motion) vary from this layout, so consult the Bentley or Haynes manual for specifics on those.

Catalytic converter failures are all too common in these cars, usually resulting in the dreaded DTC P0422 or P0432 "Catalyst Efficiency Below Threshold" code. Catalytic converter failure can occur for a myriad of reasons, usually including (but not limited to) a combination of the following:

1. Oil overfill or leakage into combustion chamber
2. Bad spark plugs, wires, or ignition coils (incomplete combustion)
3. Misfires
4. Bad fuel
5. Damage from fuel additives
6. Bad upstream O2 sensors
7. Physical damage
8. Bad coolant temperature sensor (CTS)
9. Short trips (low cat temps)
10. Bad Mass Airflow Sensor (MAF)
11. Bad Secondary Air Injection Pump (SAIP)
12. Old age and high mileage

Any of those items look familiar? If you've been around these cars for long, you've undoubtedly seen many of them mentioned. With any catalyst work, it's imperative to find the root of the problem unless you plan to do this job on a regular basis! In my case, a combination of old age, short trips, and old spark plugs were the most likely culprits. By the time I finally got a CEL warning, the cat internals started rattling almost immediately. Since I don't have to face emissions inspections where I live, I decided to take my time and assess all of my alternatives. In this writeup, I simply removed the cat, gutting the remaining (non-functional) bits rattling around inside, and reinstalled it. This effort started as a "dry run" for actual replacement—I do plan to make the proper replacement in the near future. A gutted cat can be pretty loud at certain RPMs and it's definitely stinking up my garage…and contrary to popular rumor, gutting most modern cats won't really help with power much, if at all (in many cars, it actually hurts some of the powerband by creating a more turbulent airflow than existed before gutting!).

Your options for Catalyst replacement are:
1. Universal converters ($50-$150) — your old converter is cut out and the universal cat is welded into place. This is a very common procedure at your local muffler shop, but the universal converter is often a substandard part…and the warranty offered typically doesn't cover labor for very long, so you might end up back in the shop, paying even more labor cost the second time around.

2. Direct fit converters ($275-$400+) — these bolt into place with no cutting or welding. The part is much larger and costs more than a universal, but is much closer to the original stock part. You can also walk into your dealership and buy an actual OE cat, but plan on spending $1,000 or more for it.

Please note that if you live in California, your options for both of the above are very limited and generally more costly.

Difficulty: Harder than a brake job, but easier than control arms or most suspension work.


  • Ramps (highly recommended over jackstands)
  • Phillips-head screwdriver
  • Penetrant spray (PB Blaster recommended)
  • Ratchet extension set (Wobble type recommended—Harbor Freight, $10 for a set of 12)
  • O2 sensor socket set (~$30 or loaned from auto parts store)
  • Ratchet set with 13mm and 17mm sockets
  • 13mm wrench
  • 13mm stubby wrench (or small 1/4" drive 13mm socket)
  • Wire cutters or blade
  • Flashlight or headlamp/worklamp
  • Jackstand for supporting exhaust pipe
  • Breaker bar


  • Direct fit catalytic converter of your choice (Emico, Bosal, DEC, etc)
  • Exhaust gasket for the manifold-to-downpipe connection (optional, recommended)

Instructions for Catalytic Converter for the driver's side (Bank 2). The other side is roughly a mirror image, but requires some moving of the car's intake parts to gain access to the O2 sensor.

1. Lift the car as high as you safely can. You can use jackstands, but ramps are far safer, quicker, and easier. Since my car is lowered, I used decking wood to build a pair of ramp extensions/boosters to raise the car another 3". I could have done the job with just the ramps themselves, but the work quarters would have been much tighter.

2. Remove coolant expansion tank (and sensor) and set aside.

3. Locate and gain access to upstream O2 sensor, which is just behind the exhaust manifold. Spray the sensor and manifold bolts with PB Blaster. Trace the sensor wire to the firewall and unplug the harness. This requires working around a spaghetti of other sensor wires on the firewall, so just move things around until you can get your hands (and the O2 socket) onto the sensor. It helps to find the wire that leads to the O2 sensor you're removing, since freeing that wire will allow the sensor to spin freely as you unscrew it. Many of these wires are bundled together with zip ties at the factory.

4. Remove upstream O2 sensor and set aside.

5. Get under the car and place your head just inside the left front tire. From there, you should be able to see the manifold bolts. It can be helpful to remove the car's belly pan, but it shouldn't be necessary.

6. The bottom two nuts are pretty easy with a ratchet, but the top nut has very little clearance for regular tools. This is where the 13mm stubby wrench or mini ratchet comes in handy! Learn from my errors.

7. Once the nuts are free, the exhaust will be resting on the remaining bolts. Now it's time to move backwards to the other two connections.

8. Follow the exhaust pipe all the way back, past the cat, to the sleeve hangers about halfway down the car. You only have to remove the two nuts on the side you're working on. These bolts are fully exposed to road crud and salt, so they may require some PB Blaster, patience, and a breaker bar. Once you've got them loosened a few turns, slide the sleeve backwards towards the resonator.

The rear cat pipe should now be free!

9. Support the rear cat pipe with a jackstand, block of wood, or have a helper do it by hand. Now remove the rear O2 sensor with the special socket. This will create a torsion on the bundle of wires as you unscrew the sensor, but shouldn't cause any damage.

When you reinstall the sensor, you can "pre-twist" the wires to offset this effect.

10. Now remove the middle exhaust hanger, which is a tension mounted spring just before the flex pipe. A wrench and ratchet should make quick work of it, depending on rust.

11. Your cat is now completely free of the car. Gently work the cat assembly backwards until the manifold flange is out from the engine compartment. Be very careful not to put too much pressure on the flex pipe if you ever plan to re-use the assembly! It can crack and split with more than ~10 degrees of movement.

12. Reinstallation is fairly simple from this point. Don't forget to use anti-seize compound on the O2 sensors. Be sure to keep all exhaust connections slightly loose to allow for fitment adjustments. Don't forget to replace the manifold gasket! The manifold needs to be tightened first, giving a couple of turns to each nut and going around the outside of the flange to ensure an even seal. Then you can torque down the other connections in the back. Start the car and check the connections before you back it down off the ramps—any exhaust leaks at the manifold should be pretty obvious! It took me 4-5 hours of trial and error (and a blackened, swollen pinkie finger) to figure out what works and what doesn't, but next time around will be less than half of that.

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