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Discussion Starter #1
My 2000 V6 4-motion variant has been throwing codes P0421 and P0431. I've been trying to figure out if the cats are really bad.

According to Bentley, there should be an O2 sensor behind both cats. Well, I got underneath it yesterday and it has O2 sensors just behind the exhaust manifold but the rear sensors are in FRONT of the cats. Is Bentley wrong on this one? If the sensor is in front of the cat then how could it possibly monitor whether the cat is working?
 

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I am dealing with the same problem. The cats are covered under warranty if it has less than 80k. The dealer ordered a new set. I am wondering if I should change the O2's as well. I've read you should around 60k. I wonder how you would know if the O2's are bad. Would they spit out their own codes?
 

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Here's what ETKA shows for your 4MO (also see more below pic):



Testing a cat is a fairly straight forward process. Check out this Power Point Presentation from NGK (cut and paste into browser): home.att.net/~ngksparkplugs/Oxygen_Sensor_Tech_Review.ppt

And this should help:

Testing O2 sensors that are installed:

The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedence DC voltmeter to the Oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do. Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire. When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check your leads to make sure you won't wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes the sensor as hot and active once there are several cross counts. You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high (> 0.45) near 0.45 or steady low (< 0.45). If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the reading is steady low, add richness by partially closing the choke or adding some propane through the air intake. Be very careful if you work with any extra gasoline, you can easily be burned or have an explosion. If the voltage now rises above 0.7 to 0.9, and you can change it at will by changing the extra fuel, the O2 sensor is usually good. If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of it's hose and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good. If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor. If you are not getting a voltage and the car has been running rich lately, the sensor may be carbon fouled. It is sometimes possible to clean a sensor in the car. Do this by unplugging the sensor harness, warming up the engine, and creating a lean condition at about 2000 rpm for 1 or 2 minutes. Create a big enough vacuum leak so that the engine begins to slow down. The extra heat will clean it off if possible. If not, it was dead anyway, no loss. In either case, fix the cause of the rich mixture and retest. If you don't, the new sensor will fail.

Testing O2 sensors on the workbench.

Use a high impedence DC voltmeter as above. Clamp the sensor in a vice, or use a plier or vice-grip to hold it. Clamp your negative voltmeter lead to the case, and the positive to the output wire. Use a propane torch set to high and the inner blue flame tip to heat the fluted or perforated area of the sensor. You should see a DC voltage of at least 0.6 within 20 seconds. If not, most likely cause is open circuit internally or lead fouling. If OK so far, remove from flame. You should see a drop to under 0.1 volt within 4 seconds. If not likely silicone fouled. If still OK, heat for two full minutes and watch for drops in voltage. Sometimes, the internal connections will open up under heat. This is the same a loose wire and is a failure. If the sensor is OK at this point, and will switch from high to low quickly as you move the flame, the sensor is good. Bear in mind that good or bad is relative, with port fuel injection needing faster information than carbureted systems. ANY O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, AND pass the two minute heat test is good regardless of age. When replacing a sensor, don't miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always *no* benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph.
 

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Discussion Starter #4
My rear O2 sensors are definately not where the picture shows. They are at the inlet side of the cats. I'm not sure what this means. I'm the second owner and the car has a lot of miles on it. Who knows, maybe someone put the cats in backwards.

Thanks 1badWGN, I'm on information overload now. :bow:

BTW, I have a VAG-COM and have run some diagnostics. The readiness codes checked out good for the O2 sensor tests. Measuring block 030 did not read the required 111 value for the sensors behind the cats. Even after warming it up for a while it still read 110. I could not get the 3-way cat test to run.
 

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Discussion Starter #6
Thanks Trekie. I'm starting to think something is wrong with the way my exhaust is installed.

Anyone out there with a 2000 V6 4mo who can verify the rear O2 sensor location?
 

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nomow,
what's your engine code out of curiosity? i want to make sure that the picture matches your engine code for sure. I've checked 4MO and the 2WD V6 and both pics show exact same place.

you have to have your O2 sensor behind the CAT. there has to be one before and one after. that's a requirement of OBDII. if it's not there, it's way beyond me.
 

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Double checked. The picture reamins the same for ATQ. It supposed to be there. Have you gone hand over hand down the entire exhaust? Is your front one where the picture shows?
 

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Discussion Starter #10
The front sensors are in the right location as per the picture. I have checked out the exhaust from the engine to behind the cats. I'm going to try to get a photo of my system tonight.
 

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Discussion Starter #11
Well here they are. This view is from the front of the car. Both sensors are at the inlet side of the cats. The passenger side one is on top (circled).
 

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You know what? Check up by your exhaust manifold for another set. By looking at your picture, these are a little after the flex pipe. This may be the rear set because it's right in the CAT. Check it out, let me know. It still puzzles me how the diagrahm is wrong though.
 

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Discussion Starter #13
I think the diagram must be wrong. For a while I was thinking someone may have replaced the cats but my exhaust looks to be stock. I would like to have a look underneath a new one sometime just to verify it.

I checked the car out some more and I'm pretty much convinced that the cats are bad. I've seen quite a few posts about bad cats. VAG-COM is showing everything is ok with the O2 sensors and not ok with the cats. I will have to take it to a shop to have the pre and post cat temps measured to be sure. They don't have any rattles and the car runs fine so I don't think there is any blockage.

Any ideas on aftermarkets cats? $3000 for OEM is out of the question.
 

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Here's the lowdown. The V6 Passats do not have 2 catalytic converters, they have 4! I took it to my mechanic to have him look it over and there are two small cats just behind the front O2 sensors. I guess I missed those when I was checking it out. :oops: The cats with the O2 sensors in them are the rear ones.

I checked the VW site and the engine info for the V6 shows two pre-cats and two mains. I'm not sure why Bentley and ETKA show the wrong info.
 
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