# 1.8T O2 Sensor question - P0420 code



## ebjork (Jul 28, 2008)

Yeah my CEL just came on last night.. pulled the code it is a P0420 - Catalytic Efficiency Below Threshold Bank 1. I am guessing it is the rear sensor. Without running any VAG tests, I am planning on picking up a rear sensor today, and doing an install. I have stock cat and DP. Is there any chance it could be the front sensor and I could be wasting time, money, and effort here? anyone have any run-ins with this problem before? All help very appreciated. Thanks.


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## veedubwolfsburg (Jun 4, 2007)

*Re: 1.8T O2 Sensor question - P0420 code (ebjork)*

I doubt it, as your front sensor wouldn't have any idea as to how well your catalytic converter is functioning.
I used to get this code regularly, but I haven't seen it since I've been chipped.


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## O2VW1.8T (Jul 9, 2003)

*Re: 1.8T O2 Sensor question - P0420 code (veedubwolfsburg)*

Its a bad cat, replace the cat


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## Metalingus (Jan 4, 2008)

i have this code to








i hope its only the rear 02 sensor and not the cat..


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## dubbboy18t (Feb 12, 2008)

i had the same code come up .. i got rid of my cat .. with a new dp .. and even with a spacer to code comes on . thinkin it might be a badd o2


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## EuroTrash88 (Oct 4, 2007)

*Re: (dubbboy18t)*

its either ur cat is shot or ur o2 is shot. if its ur cat, u will hear alot of rattling from the exhaust under heavy accel, happened to me on my galant at 32k. if its the o2, screw it, the rear o2 is only to measure cat efficiency, not regulate a/f. i would replace when inspection rolls around if ur strapped for cash.


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## GLiMKIV (Nov 12, 2006)

*Re: 1.8T O2 Sensor question - P0420 code (ebjork)*

8 YR/ 80K warranty on stock converter.


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## jazzblueturbo (Jul 29, 2008)

the best fix for a p0420 code or anyone running no cat/dp with high flow cat....buy spark plug anti-fowlers. use both of them on top of eachother to space out your rear o2 sensor. youll never throw a cat code again and your readiness will set as good so youll pass inspection....anti-fowlers sell at pepboys and autozone for $5.00


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## Metalingus (Jan 4, 2008)

i have till may of 09 i think to get my next inspection so i might just ignore it. if it is the cat is shot, is it bad to keep driving the car?


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## GLiMKIV (Nov 12, 2006)

*Re: (Metalingus)*

If the converter is plugged it will cause you problems.


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## EuroTrash88 (Oct 4, 2007)

*Re: (Metalingus)*


_Quote, originally posted by *Metalingus* »_i have till may of 09 i think to get my next inspection so i might just ignore it. if it is the cat is shot, is it bad to keep driving the car?

well if the cat is shot u will notice it immediately. gas mileage plumets, u get a huge lose of power. and if the cat has ripped itself to bits, like mine had done, under heavy accel exhaust will make a very loud raspy sound cause the chunks of the cat are jus flying around in there at high velocity. but if the cats fine and only o2 is shot, id say leave it, u'll be good.


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## dubbboy18t (Feb 12, 2008)

yea today ... ok so last night after doing the diode i was under my car lookin at why my cel was on .. had to be something up with the dowpipe and o2 .. removed the o2 spacer and put the o2 directl on the downpipe again .. so far no codes .. its weird like when i put the o2 spacer in ..within a little bit of diving it goes on .withought the spacer lasted almost 2 days and still counting .. "knock on wood " .


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## jazzblueturbo (Jul 29, 2008)

the computer wont put the p0420 code on until the vehicle has gone through the appropriate number of complete drive cycles for the emmissions system. a drive cycle is a weird serious of driving at diff speeds and rpms and temps. if you have a scan tool and you monitor your readiness of the state inspection modules, youll know the drive cylces have completed becuase all the modules will say "ready" if they say "inc" or "not ready" than you havent hit that magic number of driving conditions yet....once it took me 5 weeks. another time it took me 35 miles.


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## jazzblueturbo (Jul 29, 2008)

oh i forgot to add that a bad 02 will almost Never set a cat efficiency code. to set a cat effeciency code the cat HAS to be doing its job. if the o2 goes bad the computer would never know if the cat was good or bad. it most likely would set an o2 code...either high imput, low imput, high/low voltage, or even an 02 heater code for the front sensor.


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## Salsafart (Mar 24, 2008)

*Re: (jazzblueturbo)*

i was wondering... im getting a highflow cat and a 3 Inch downpipe for my 1.8t and i know that setup is notorious for running p0420 codes, could u explain to me about the plug anti fowlers???


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## renegdewolf (Aug 12, 2007)

*Re: (Salsafart)*

I was getting the 90420 code so I was planning on a new exhaust anyway and since I'm going BT I opted for the Maxpsi 3 in turbo back. I don't have any codes


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## 04boostedGTI (Aug 4, 2008)

*Re: (EuroTrash88)*

i had the code pop up and turned up being that my stock dp flex molding blew out completely. i got a new exhaust with the cat delete setup on the way. i was thinking of adding an a/f meter where would i go about doing that?









_Quote, originally posted by *EuroTrash88* »_ the rear o2 is only to measure cat efficiency, not regulate a/f.
 
now i've heard before that the rear is where the a/f plug is supposed to go.


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## EuroTrash88 (Oct 4, 2007)

*Re: (04boostedGTI)*

to clear things up, when i say rear o2 sensor i meant the one after the cat itself. literarly closer to the rear of the car. srry for the confusion


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## 04boostedGTI (Aug 4, 2008)

*Re: (EuroTrash88)*

i know that. but could i go about adding a a/f meter at the rear o2 plug?


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## EuroTrash88 (Oct 4, 2007)

*Re: (04boostedGTI)*

if ur gonna go and throw a narrow band gauge, dont bother wit it, jus gonna waste ur 50 bucks gettin the thing and doin all the work to wire it. narrow bands dont give u accurate readings. if ur gonna get a wideband gauge and jus connect it directly onto the o2 sensor, go for it, but note that the cel will be on for failed o2 sensor since u will have to remove the wires goin to the ecu, and u wont be gettin good readings since its after the car. if u decide to go and splice into those wires instead and get reading on ur gauge. dont bother wit that cause splicing in the gauge will increase the resistance on the o2 sensor and thus give u false readings again. only way is get a standalone wideband o2 by aem and hook it up b4 the cat.


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## GLiMKIV (Nov 12, 2006)

*Re: (EuroTrash88)*


_Quote, originally posted by *EuroTrash88* »_if ur gonna go and throw a narrow band gauge, dont bother wit it, jus gonna waste ur 50 bucks gettin the thing and doin all the work to wire it. narrow bands dont give u accurate readings. if ur gonna get a wideband gauge and jus connect it directly onto the o2 sensor, go for it, but note that the cel will be on for failed o2 sensor since u will have to remove the wires goin to the ecu, and u wont be gettin good readings since its after the car. if u decide to go and splice into those wires instead and get reading on ur gauge. dont bother wit that cause splicing in the gauge will increase the resistance on the o2 sensor and thus give u false readings again. only way is get a standalone wideband o2 by aem and hook it up b4 the cat.

Amen!! http://****************.com/smile/emthup.gif


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## Metalingus (Jan 4, 2008)

hmm i think ill just leave it alone for now, ive driven probably 400 miles with it and is still fine so whatever.


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## 04boostedGTI (Aug 4, 2008)

*Re: (EuroTrash88)*

thanks, i think i might just let be for now until i get my car track ready.


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## mocas (Aug 31, 2005)

*Re: (04boostedGTI)*

I have had this issue for a year changed rear 02 sensor, cat downpipe still came on. Now i have a full 3" turboback with highflow cat and it comes on way more frequent car is chipped too angle block is next on the list.


_Modified by mocas at 1:43 PM 9-9-2008_


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