[smitty40]

My 2002 V6 ATQ (191K 29MPG Highway 23 City) was due for state inspection so I needed to come up with a fix for my P0421 P0431 CEL codes. I planned to replace both rear O2 sensors, but first wanted to be sure this wasn't due to poor combustion performance or an exhaust leak. So I put 1 bottle of Techron fuel injector cleaner into a 1/2 full tank of premium, then hooked up my shop vac to one of the two the exhaust tips and plugged the other tip to determine if there was a noticeable air leak in the exhaust. I ran the vac about 1 minute, but the air would only force its way out the other exhaust tip which suggests I don't have a leak. The next day I drove the car and noticed improved throttle response and idle. After 2 more cycles of warm up and cool down the CELs turned off. Yesterday I took it in for state inspection and it passed.

Since I've used Techron multiple times before and never really noticed an immediate performance improvement, I think running the shop vac in the reverse direction of the exhaust flow may have loosened up some crud in the cat which may have at least temporarily improved the efficiency of the cat. I'm 100% sure either the Techron or shop vac leak test caused my CEL to turn off, but I don't believe Techron would have such a big impact after having it in the tank for such a short period of time. I would be interested in knowing if anyone fixed a P0421 + P0431 after using a shop vac to test for exhaust leaks, or after using Techron in the grey bottle. I will update this post with any changes in a few months.

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P0421 Warm Up Catalyst Efficiency Below Threshold Bank 1

P0421 code means that the vehicle's control module has detected that the catalytic converter is not working properly as in it is not as efficient as the factory is expecting. Replacing the oxygen (O2) sensors may sometimes fix the code, and in most cases the catalytic convert needs to be replaced to fix the problem. The P0421 code is set when the Engine Control Module (ECM) detects rear O2 value difference becomes closer to value of front O2. The ECM monitors when vehicle speed is 25-60 MPH, load is 22-45 percent and engine speed is 1000-3000 RPM.

 

[GrandpaBill]

Hello Smitty,
As you noted, 421 usually means your cat is dead (still, no dead cat icon. What's wrong with you people!); 191k miles, afterall. You may simply have an intermittent 421. I'm surprised Texas MV inspection didn't discover the CELs. Does Texas conduct inspection scans? Can't explain you're experience with Techron and a vacuum cleaner hose, though I give you high marks for creativity. Techron, in my experience, is a useless product.

 

[GearHd6]

I had the 421 and 431 codes which is the warm-up catalyst code. I added two O2 sensor extenders and the check engine light has been out for a few thousand miles now and it breezed right through state emissions.

 

[PZ]

It's not the Techron. I used 2 bottles on every oil change on my V6. I had O2 spacers to keep the cat codes at bay. The shop vac might have helped, but it will be temporary.

 

[kutcht1]

Isn't there usually an overlying issue leading to a cat converter to lead to not be as effective? Driving to many miles with an engine not at optimum like running rich/lean for example. Many times replacing a converter or doing the O2 extenders are just a band-aid for another issue but very effective as I have also done it.
TomK

 

[PZ]

Yes, usually a long term misfire or bad thermostat will degrade a cat. On my V6, it was part of the software update that VW had for limits set too low (after they had spent a ton of money on in-warranty replacements). Since I was over the 100K limit for a free software upgrade, the spacers were cheaper.

On older cars (without a MAF), we used water injection (often poured right into the carb) to steam clean both the cats and carbon buildup in the cylinders.

 

[ylwagon]

On older cars (without a MAF), we used water injection (often poured right into the carb) to steam clean both the cats and carbon buildup in the cylinders.

My friend the mechanic knew all the old tricks like that. Once a woman drove in with a barely-running car looking to get it diagnosed. Joe had lots of other cars to work on that day, so he removed her air filter cover over the carburetor, then had his helper hold the throttle wide open while he poured a container of water right into the howling carb's intake. Smoke and carbon dust blew out the tailpipe, then he signaled the helper to let the engine idle, which it did nicely. He put he filter cover back on, closed the hood, and told her "no charge".

 

[Mikey319]

2002 ATQ V6 here and I only get a P0421 when my oil is to low

 

[Mikey319]

Also, my car runs like complete crap on regular. Volkswagen recommends premium for it anyway.

 

[Treppmit]

I'm having the same issue with my B5 V6 Passat. I was considering changing the O2 sensors. Can you give me a good source for the spacers?

 

[Treppmit]

That's what I had been thinking and assumed that if I corrected my random cylinder 1 misfire that keeps coming back it might get the P0421,P0431 codes to drop off. I have not been able to correct the misfire though.

 

[jjpark]

I just saw ran a scan last night and it's showing the same issue but on Bank 2 P0431 Intermittent

04 V6 GLX 172K

The stainless (or what I think is stainless) wrap/ braiding that goes around the passenger side flex pipe has completely separated at one end. I think I hear a very slight exhaust leak and have noticed it for some time but this is the 1st time I have gotten the code. No check engine light.

Is this indeed the flex pipe wearing and could this be causing the code?

Thank you

 

[smitty40]

Update from original post last month. Today my CEL came back on, so whatever it was that caused it to turn off only lasted 40 days. This could very well be a small leak in the flex pipe, so that is what I will look at next.

 

[smitty40]

Another update from original post from March 2017. My CEL seems to turn on and off depending on my driving habits. The more aggressive I drive the more likely P0421 or P0431 will turn on. I was commuting 100 miles a day on highway roads from Jan 2017 until Jan 2018 while averaging 60+ mph. As a result I was consuming a lot of oil, as much as 1/2 quart every 2000 miles and my CEL would turn on an off at random. Mikey319's post above seems to be spot on. He said "his 2002 ATQ V6 only gets a P0421 when his oil is to low." As of January my commute is only 20 miles a day and I average 30-40mph. I no longer reach highway speeds or accelerate fast from a stop. Since January 2018 my CEL has not come back on. I'm currently at 210K miles and the car still runs and drives great and it just passed state inspection. I now check the oil every week and top it off when needed, but I don't burn nearly as much oil anymore and when I top it off its only a couple of ounces of oil. Bottom Line: I now believe Mikey is right in that the oil level or amount of oil consumed could be throwing these intermittent codes.

 

[PZ]

My CEL was temp dependent. It would come on with cooler weather (what we call winter) and stop by summer when it had to be inspected. One summer it came on during a trip to Maine and stopped on the way back south.

As for oil consumption, I burned oil on high speed long distance drives (80mph for 12-16 hours) but very little around town. On my 5000 mile vacations, I would burn almost 3L. Once the heads were rebuilt with new guides and seals, the oil burning dropped to less than 1L on a 5K trip. My son's V6 does not burn as much as mine did and has not had any engine work done with 213K. It had the updated software installed before we bought the car and has not had any cat related codes.

 

[13ish]

I had both CAT CEL codes when i first got the car, fitted the rear O2 sensor spacers and no codes ever since. It always passed state inspection. Nice to hear the oil burn rate from PZ - very reassuring!