I'm having the same issue with my '03 2.8 v6 but only with the P0421 code. It's been throwing up the code for a few months now. It will come back for a few weeks then will go away for a while. Cold start doesn't necessarily seem to be what triggers the code. I also have notices a slight shutter at idle but haven't noticed any other performance problems. I've read the code with a scan tool and it says "P-421 Warm Up Catalyst Efficiency Below Threshold Bank 1". I've read all of the threads as well and am not sure what to do with the '03 as far as troubleshooting. Was going to replace the O2 sensor but wondering what you found out?W

 

well, i have the same car, same year, and the exact same thing happening!! only p0421 as well.
cel would come on about every week or two before, and after a MAF cleanup i didnt see it for 2 or 3 months. then yesterday it was back again.

 

I had P0431, P0421 and random misfire codes.

Replaced ignition coil - My misfire codes are gone
Replaced front and back Oxygen sensor for driver side - P0431 gone for 2 weeks. Hope it will not come back.

Planning to replace Passenger side Oxygen sensors - These 2 are lot more harder than driver side as per the posts here. Hope it will clear the code P0421.

I remember dealer changed the CATs almost at the end of emission warranty probably around 2007/2008. Hoping CATs are ok.

- mmdas
2002 Passat V6 Wagon 4Motion 137k miles

 

I had a similar problem which led me to join this site. My mechanic actually gave up so I had no choice!

All started when I blew a VCG. They replaced it, and put in new non VW plugs. then the random misfires started along w cat related codes. They told me to go to stealership.

At this point I still knew nuthin about cars. The stealership was quick to tell me the car was misfiring (no shit i thought) BUT they also kindly pointed out a piece of tubing I eventually learned was part of the PCV system. They also said to buy VW plugs.

I replaced the plugs, things improved then worsened. Later, we saw a purple arcing that led me to replce the spark plug wires. Still misfiring. A friend who worked on older cars and didn't know about PCV system said he thought the compromised tubing needed to hold vacuum (he was cluey) so we replaced w $6 heater hose and hose clamp. Things improved but then misfires returned. Husband told me to start looking for another car.

But I perservered. I called the stealership for the codes- they didn't record them BUT they did say both bank precar sensor. I thought wht the heck, $40 a piece on Amazon for Bosch universals tht fit my car; hired the tool from autozone. Pulled the car to the alley (barely made it, banging and popping misfiring all over the shop). Once I stuck them in (thanks to the users on this forum who put info here) car was TOTALLY fine- even CEL went off!! All 11 codes satisfied.

I knew by this stage the reason I blew VCG was the PCV. I could see around VCG had "white fumes" leaking up so I went in and cleaned out PCV system, replaced the short back piece even cuz it broke (bloody hard get back there!!!) and Oil air separator (cracked it open and saw it was compromised). After putting it all back together, no
more white fumes.

The early symptom for all this was a constant need for oil- I had asked the
mechanic 2 years earlier and they said they couldn't tell without tearing down- now I know if PCV is compromised,oil goes to the "weakest" parts of the engine including your seals and gaskets. When I blew the VCG the o2 sensor(s) were fouled too; this as I understand causes issues regulating air:fuel so yr passat misfires. For the price, I replaced both pre cat sensors.

I'd check out yr PCV system if not yet done that, even if u replace the sensors and that fixes yr problem- the DIY for my 2002 v6 (B5.5) I believe was somewhere on the forum. Tip: Get part#s from stealership and google them, sometimes random sites have the parts cheaper - eg rear PCV hose was $9.99 on one site (unfortunately out of stock- had to fork $110 at Stealers).

Sorry this was long, but I wanted to get this experience on the forum since I was THIS close to needing a new car!

I am just glad it wasn't the post cat sensors; not game to get under the chassis at this point!

 

Get part#s from stealership and google them, sometimes random sites have the parts cheaper

2003 V6 ATQ PCV Part numbers... Check to see if your system looks like the diagram with the UFO looking piece.
http://www.passatworld.com/forums/4...b5-discussion/341170-passat-b5-5-v6-atq-pcv-valve-part-numbers.html#post2834163