Fix for P0421 - Warm Up Catalyst Efficiency Below Threshold Bank 1

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  • 3 Post By ylwagon

Thread: Fix for P0421 - Warm Up Catalyst Efficiency Below Threshold Bank 1

  1. #1
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    Fix for P0421 - Warm Up Catalyst Efficiency Below Threshold Bank 1

    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.

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  3. #2
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    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.

  4. #3
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    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.

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  6. #4
    PZ
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    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.

  7. #5
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    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

  8. #6
    PZ
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    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.

  9. #7
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    Quote Originally Posted by PZ View Post
    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".
    PZ, John_E and jjpark like this.

  10. #8
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    2002 ATQ V6 here and I only get a P0421 when my oil is to low

  11. #9
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    Also, my car runs like complete crap on regular. Volkswagen recommends premium for it anyway.

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