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Ok so first and foremost the car is a 2004 Passat V6 with 190,000 miles on it. I have to go through emissions testing in a month and have a problem resulting in check engine lights which will make me fail.

The CEL’s are as follows: P0058, P0161, P0421

P0058 and P0161 indicate a problem with O2 sensor Bank 2 Sensor 2. The sensor was replaced and the codes came back. I also installed an 02 sensor spacer about 100 miles ago to try and fool the ECU but the codes persisted.

The P0421 only comes on once every couple of weeks.

I am trying to put as little money into the car as possible so any help troubleshooting would be greatly appreciated. The car has no had a drop in gas mileage or performance.
 

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The first two codes relate to the sensor heater circuit, so adding a spacer to "fool the ECU" won't work, I believe. Assuming that your new B2 sensor has a good heater, then the problem is wiring, connectors, or even the ECU. The P0421 may just be the result of an old cat. My 3.0 A4 at 192,000 miles is giving me those warm-up efficiency codes now and then, and I figure that the cats are just becoming less effective.
 

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The first two codes relate to the sensor heater circuit, so adding a spacer to "fool the ECU" won't work, I believe. Assuming that your new B2 sensor has a good heater, then the problem is wiring, connectors, or even the ECU. The P0421 may just be the result of an old cat. My 3.0 A4 at 192,000 miles is giving me those warm-up efficiency codes now and then, and I figure that the cats are just becoming less effective.
Any pointers or tips for troubleshooting the wiring or connectors? Is there a way to test the ECU?
 

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To start with, you should make sure the sensor's heater circuit is complete up to the connector. I saw this which might help:

http://www.vaglinks.com/Docs/Audi/Misc/AudiWorld.com_Audi_B5_C5_O2_Sensor_Testing_DIY.pdf

If that looks OK, then I would remove the cover from the ECU box, and (with the battery disconnected!) disconnect all of the ECU connectors, and spray both pins and receptacles with electrical contact cleaner. I do this a matter of routine every other year or so, with every engine connector, just to avoid connection issues. While disconnected, check continuity from the O2 sensor connector to the respective receptacles at the appropriate ECU harness connector. You will need to track down an ECU pinout to know which ones are for that sensor.
 

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Searching a P0421 code in the forum brings up an endless list of stuff that is vague at best.

The early V6's had issues with software that was overly sensitive and gave false readings and sent scores of unsuspecting people scurrying to the stealership to have their cars misdiagnosed and a lot of people paid very dearly for brand new cats when there was never anything wrong with to begin with.

An update was finally issued (TSB# 01-05-13) but never an official recall.


Seeing as your car is a 2004 it does not fall under this category. Unless it has an early model ECU (and never flashed with the update) that was changed out at some point in time.

Do you have VCDS?
Also, do have any misfires (codes) or burn a significant amount of oil between oil changes?

P0421 seems to draw a general consensus that a bad cat is the reason for this code. I'm not sold on that, unless your pouring in a quart of oil every week to keep the oil level at it's required level.

The first and cheapest thing you can do is clear the codes with VCDS and disconnect the back (post cat sensors) and see if the code comes back. On the firewall, you want to disconnect the brown and green connectors. They are on each side of the engine right at the rear of the valve covers.
This is not the proper fix to your problem but it might allow to pass the emissions test to get you going down the road.

The next thing to do is check all the wiring and see if the wires have any cuts or somehow have made contact with the exhaust pipe and melted the wires.

If your not burning oil and don't have any misfire codes, then I guess the next thing would be to purchase a new post cat 02 sensor and put that in.



If anybody else has got anything worth chiming in here, I'm all ears.
 

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I just bought these from Rock Auto 3 months ago for my 98 GLS v6.

Genuine OEM with factory colored connectors $68 a piece, cheap!

BOSCH 13552 OEM sensor
Downstream Left

BOSCH 13550 OEM sensor
Downstream Right
 

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Discussion Starter #8
Searching a P0421 code in the forum brings up an endless list of stuff that is vague at best.

The early V6's had issues with software that was overly sensitive and gave false readings and sent scores of unsuspecting people scurrying to the stealership to have their cars misdiagnosed and a lot of people paid very dearly for brand new cats when there was never anything wrong with to begin with.

An update was finally issued (TSB# 01-05-13) but never an official recall.


Seeing as your car is a 2004 it does not fall under this category. Unless it has an early model ECU (and never flashed with the update) that was changed out at some point in time.

Do you have VCDS?
Also, do have any misfires (codes) or burn a significant amount of oil between oil changes?

P0421 seems to draw a general consensus that a bad cat is the reason for this code. I'm not sold on that, unless your pouring in a quart of oil every week to keep the oil level at it's required level.

The first and cheapest thing you can do is clear the codes with VCDS and disconnect the back (post cat sensors) and see if the code comes back. On the firewall, you want to disconnect the brown and green connectors. They are on each side of the engine right at the rear of the valve covers.
This is not the proper fix to your problem but it might allow to pass the emissions test to get you going down the road.

The next thing to do is check all the wiring and see if the wires have any cuts or somehow have made contact with the exhaust pipe and melted the wires.

If your not burning oil and don't have any misfire codes, then I guess the next thing would be to purchase a new post cat 02 sensor and put that in.



If anybody else has got anything worth chiming in here, I'm all ears.
Thank you so much for the reply! Drivers side rear 02 sensor was replaced when I first started troubleshooting this issue which did not clear the issue with the P0141, and P0058. The P0421 is intermittent at this point. No changes in gas mileage or performance. I do not have access to VCDS but no other codes according to an OBDII reader. Car is burning no oil between changes. Mind filling me in on the location of the ECU so I can check continuity? I really appreciate your time.
 

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The ECU, aka computer, is in the black plastic box below your driver's side windshield wiper. ECU is contained in an Aluminum case, but only disconnect wiring or touch the connectors after discharging yourself on the car's grounded meta.
 

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Discussion Starter #10
The ECU, aka computer, is in the black plastic box below your driver's side windshield wiper. ECU is contained in an Aluminum case, but only disconnect wiring or touch the connectors after discharging yourself on the car's grounded meta.
Thanks! Now I just need to find an ECU wiring diagram so that I can check continuity. Any ideas on where to find one? My search function isn’t finding much.
 

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The schematics are in the Bentley manual (~$95), but it's copyrighted, and the schematics go for a few hundred pages (for various engines, model years, etc), so you won't find it online.

The heater wires are on pins 1 & 2 of the oxygen sensor connectors. Pin 1 is green/yellow, the +12V connection. Pin 2 varies per sensor; it's the controlled ground side, switched by the ECM. These wire colors are on the ECM side of the connectors.

On the sensor side of the connector, you'll likely find that pins 1 & 2 have the same color of wires. The heater is not polarized, so they're interchangeable as far as the sensor is concerned. Pins 3 & 4 will each have unique colors.

I'd suggest you unplug the offending sensor and measure the resistance between the two heater wires (on the sensor side), as well as resistance to ground. The resistance between the heater wires should be roughly 10-15 ohms. The resistance to ground should be an open circuit/infinity. If you have doubts, measure one of your good sensors; they should be roughly the same.

That should let you pretty easily identify whether you have a sensor issue (or wiring issue, if a generic sensor was spliced in). If you do have a wiring issue, it's very likely on the sensor side of the connectors. The wiring between the connectors and the ECM is much less prone to damage.
 

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If you plan on keeping the car for any length of time and doing your own maintenance, the Bentley manual is a must have.
It is the VW bible for Passat's published by Bentley Publishing. They can be ordered on-line.
I would also consider at least the VCDS Lite version from Ross-Tech: Home so you can scan your cars various control modules for issues and problem solving. VCDS will scan deeper than a generic OBDII reader.

My last post I jumped over the testing of the sensors, completely forgot to make any mention of it.
FrescoGreen01.5 has good sound advice on post #11, I would follow that and see what that nets you.
 
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