# P0420 Bank 1 Effic Below Threshold code keeps coming up...Cat or 02 or ??



## Jesse Treg (Mar 31, 2007)

recently I pulled code P0420, AKA Bank 1: Efficiency Below Threshold. To start from the bginning...several weeks ago my EPC light came on so, like most educated consumers I decided to buy a vag-com. I pulled the codes and got both the Bank 1 and Bank 2 efficiency below threshold codes (p0420, p0430). I reset the light and literally within 3 minutes the EPC light came back on only this time just Bank 1. So I drove around again for a week and the check engine light came on this time, back to Bank 1 and Bank 2. I am hoping someone will be able to point me in the right direction as to whether trying to replace the spark plugs will possibly fix the misfires, its Cat Converter replacement or possibly just replacement of 02 sensors. I have been reading mixed responses.
FYI I use synthetic oil and chance the oil every 7500-9000 miles, have 89,000 miles and have never changed my spark plugs.


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## depiry (Feb 16, 2005)

Which Engine do you have? Marty


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## depiry (Feb 16, 2005)

P0430 16814 Catalyst System Efficiency Below Threshold Bank 2


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## depiry (Feb 16, 2005)

P0420 16804 Catalyst System Efficiency Below Threshold Bank 1


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## depiry (Feb 16, 2005)

Check O2 Sensors,then cats,Marty


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## depiry (Feb 16, 2005)

Try Cat cleaner and new plugs,O2 sensors then cats,Marty. Bentleys procedure.


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## jdog19VR6 (Apr 25, 2006)

*Re: (depiry)*

02 sensors


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## treg4574 (Apr 14, 2004)

*Re: P0420 Bank 1 Effic Below Threshold code keeps coming up...Cat or 02 or ?? (Jesse Treg)*

No use doing anything else until you have changed the spark plugs.
They are overdue and even small amounts of unburnt fuel going into the exhaust manifold can set off an O2 sensor warning.
It is very likely you need O2 sensors too, but no point doing that until you have changed the plugs and drove it a few miles to be sure.


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## spockcat (Jun 5, 2003)

*Re: P0420 Bank 1 Effic Below Threshold code keeps coming up...Cat or 02 or ?? (treg4574)*


_Quote, originally posted by *treg4574* »_No use doing anything else until you have changed the spark plugs.
They are overdue and even small amounts of unburnt fuel going into the exhaust manifold can set off an O2 sensor warning.
It is very likely you need O2 sensors too, but no point doing that until you have changed the plugs and drove it a few miles to be sure.


And perhaps the coil packs? Maybe one or more is starting to go?


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## BenF (Feb 9, 2008)

*Re: P0420 Bank 1 Effic Below Threshold code keeps coming up...Cat or 02 or ?? (spockcat)*

I am having a similar problem with my 2000 jetta VR6. 
I get only p0420 and CEL comes on about an hour or so into driving after clearing it.
I tried replacing the spark plugs and wires (wires because the old cyl5 wire was burned to a point where it woudln't work with a new plug..) still have the same problem. The car is sort of rough in idle among other things.
i really hope i don't have to buy a new cat. Maybe i just have to feed it more?


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## VW Dealer Tech (Jun 14, 2003)

If it's a V6 there maybe a ECM flash for your engine. Your dealer can check it for ya.


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## tuntsfaah (Apr 2, 2007)

*Re: (VW Dealer Tech)*

I got the same cel p0420...my '05 GLI recently bought from Dealer w/ 27000 tripped after installing my used turbo-back w/high flow cat from my '01 wlfsbg. I was told that it was possilby my cat being clogged up.
I don't believe that my O2 sensor is bad since the car hasn't been ran and I got it fully functioning stock.
Does anyone have any insites into this problem?
I want to think it's the cat since 2 days after installing the turbo-back the cel came on.
Plus I realize that I have been burning a lot of gas too


_Modified by tuntsfaah at 9:19 AM 2-28-2008_


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## deonez110 (Nov 13, 2008)

*Re: P0420 Bank 1 Effic Below Threshold code keeps coming up...Cat or 02 or ?? (Jesse Treg)*

I have had the P0420 as well asp0102/p0113/91142.
0420-Bank 1: Efficiency Below Threshold
0102-MAF circuit low input
0113-Intake Air Temperature (IAT) Sensor 1 Circuit (G42): High Input 
1142-Load calculation cross check: lower limit exceeded
These have all lead me back to my Mass Air. After replacing it my trouble codes disappeared. I am not saying it is the answer to your problems but it is less expensive then a coil pack and easy to swap out. http://****************.com/smile/emthup.gif


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## spockcat (Jun 5, 2003)

*Re: P0420 Bank 1 Effic Below Threshold code keeps coming up...Cat or 02 or ?? (deonez110)*


_Quote, originally posted by *deonez110* »_I have had the P0420 as well asp0102/p0113/91142.
0420-Bank 1: Efficiency Below Threshold
0102-MAF circuit low input
0113-Intake Air Temperature (IAT) Sensor 1 Circuit (G42): High Input 
1142-Load calculation cross check: lower limit exceeded
These have all lead me back to my Mass Air. After replacing it my trouble codes disappeared. I am not saying it is the answer to your problems but it is less expensive then a coil pack and easy to swap out. http://****************.com/smile/emthup.gif 

LOL. 18 month old thread. Think his problem is already solved.


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## deonez110 (Nov 13, 2008)

*Re: P0420 Bank 1 Effic Below Threshold code keeps coming up...Cat or 02 or ?? (spockcat)*

Dude, haha... i realized after i posted.. thats what happens when you sit in front of a computer all day.. vision gets blurry, you hate your job, you work long hours. Sucks


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## MzLishaBoO2U (Aug 29, 2013)

*PO 420 code*

So, i know this is a REALLY old thread, but any help would be greatly appreciated!

i have a 2004 VW Jetta 2.0L 4cy

in the past few months i have noticed that the temp gauge has been tweaking out a bit, so i know that i should replace the sensor which is not a huge deal, only 10 bucks.

BUT in the past month my check engine light has come one, was reset, came on again and then turned off on its own. when a friend checked, the code i got was PO 420, saying something about the catalytic converter working under efficiency or something or other.

Now, ive been told that the first thing i should do is replace the oxygen sensor. OK. 

my only question is, if it is an o2 sensor failing, then which one should i have replaced first, upstream or downstream? is there any way to find out which one is failing? i cant really afford to replace both at the moment. my car is working fine for all intents and purposes, although it is a little jumpy sometimes. 

any thoughts? any help would be appreciated! thanks!

-little girl that know nothing about cars... :banghead:


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## rinaic (Mar 23, 2004)

You need to post this in the Jetta forum.


In a 2004 Touareg if you were brave you could try running a course or two of acetone through the system. General auto videos on YouTube how and why to add acetone and burn it out of the system.
This method is for cleaning up the cat rather than impacting the O2 sensor.

Also on a 2004 Touareg you would check the O2 sensors and then any related vacuum lines.


But I'm guessing the cats on a Jetta aren't thousands of dollars like you have on a Touareg either.


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