001056 - Catalyst system; Bank 1 P0420-002 - efficiency below threshold

[LC5F]

For the last 2 months my Golf has been throwing up a CEL for:
001056 - Catalyst system; Bank 1
P0420-002 - efficiency below threshold - Mil on

There has been no loss in performance.
CEL light only come on twice, 1st time early January and now 5 weeks later.
Previous testing in VCDS for system readiness, lambda control and lambda aging all suggest everything is OK - got screen shot's for each test.

From a thread on here it broke down possible causes:
Cat break down - My Magna flow sport cat was installed 6 months old and gave excellent figures at the late october MOT -So I am confident I can rule this out.
Exhaust leak - Pretty sure its not this.
Timing out - recent new belt and chain this should also be good.
Engine temp sensors - both coolant sensors are less than 2 years old & genuine.

This all suggests it is a failed lambda - but as there is bank 1 sensor 1 & bank 1 sensor 2 how do I pin down what one is fubar?
How come VCDS doesn't provide any breadcrumbs to say what one is misbehaving?

[ald1717]

are you sure its not the sports cat, the last 2 200 cel sports cats ive used were fine for about 3 months and then started giving the same fault, i have a spare 02 sensor spacer with a mini cat built in it that removed the code for me before i got it mapped out so you can have for free if you want, it would at least help you rule out the cat if you connect it to your 02 sensor and the eml still comes back then its nothing to do with your cat

[GVK]

Generally on any car a P420 is a duff cat, on live data where you can graph the bank 1 pre - cat and post cat 02 sensor voltages, they will follow each other if the cat is duff.

If the cat is OK, the precat 02 sensor will show the switching of the sensor where as the post cat will just be a static volltage.

[LC5F]

are you sure its not the sports cat, the last 2 200 cel sports cats ive used were fine for about 3 months and then started giving the same fault, i have a spare 02 sensor spacer with a mini cat built in it that removed the code for me before i got it mapped out so you can have for free if you want, it would at least help you rule out the cat if you connect it to your 02 sensor and the eml still comes back then its nothing to do with your cat

Thanks for the offer, I tried one of those on another VW, so its sat somewhere in the garage...emphasis on somewhere!
Where your cats still cleaning up the gases - did they fail MOT?
My old MK4 1.8t had a Milltek 200 Cel sport cat I got second hand - no issues for the 3 years it was on the Golf - I'm just surprised it is an issue for the Mk5

Generally on any car a P420 is a duff cat, on live data where you can graph the bank 1 pre - cat and post cat 02 sensor voltages, they will follow each other if the cat is duff.

If the cat is OK, the precat 02 sensor will show the switching of the sensor where as the post cat will just be a static volltage.

Thanks for the explanation GVK, that makes sense I'll get busy with VCDS when it stops pouring.

[LC5F]

Generally on any car a P420 is a duff cat, on live data where you can graph the bank 1 pre - cat and post cat 02 sensor voltages, they will follow each other if the cat is duff.

If the cat is OK, the precat 02 sensor will show the switching of the sensor where as the post cat will just be a static volltage.

Thanks for the explanation GVK, that makes sense I'll get busy with VCDS when it stops pouring.
[/quote]

Its still pouring - but dad taxi duty gave me the opportunity to test - they don't seem to be following each other:

[ald1717]

are you sure its not the sports cat, the last 2 200 cel sports cats ive used were fine for about 3 months and then started giving the same fault, i have a spare 02 sensor spacer with a mini cat built in it that removed the code for me before i got it mapped out so you can have for free if you want, it would at least help you rule out the cat if you connect it to your 02 sensor and the eml still comes back then its nothing to do with your cat

Thanks for the offer, I tried one of those on another VW, so its sat somewhere in the garage...emphasis on somewhere!
Where your cats still cleaning up the gases - did they fail MOT?
My old MK4 1.8t had a Milltek 200 Cel sport cat I got second hand - no issues for the 3 years it was on the Golf - I'm just surprised it is an issue for the Mk5

Generally on any car a P420 is a duff cat, on live data where you can graph the bank 1 pre - cat and post cat 02 sensor voltages, they will follow each other if the cat is duff.

If the cat is OK, the precat 02 sensor will show the switching of the sensor where as the post cat will just be a static volltage.

no problems with mot, it was just the free flowing cat letting more gasses through that triggered the cel, tryed a simple spacer which did nothing then bought one with a mini cat built in and that solved the cel, but got it mapped out so didnt need it anymore, if theres no drop in performance or mpg then its either the sports cat throwing a cel which unless your stage 2 or above will happen, if your secondary 02 sensor has packed up completely then a spacer wont help, but in both my cases it was the sports cat causing it, vcds should show a faulty sensor so good luck

Thanks for the explanation GVK, that makes sense I'll get busy with VCDS when it stops pouring.

[GVK]

Generally on any car a P420 is a duff cat, on live data where you can graph the bank 1 pre - cat and post cat 02 sensor voltages, they will follow each other if the cat is duff.

If the cat is OK, the precat 02 sensor will show the switching of the sensor where as the post cat will just be a static volltage.

Thanks for the explanation GVK, that makes sense I'll get busy with VCDS when it stops pouring.

Its still pouring - but dad taxi duty gave me the opportunity to test - they don't seem to be following each other:

[/quote]

Thats how it should look, i'm guessing you're getting MIL on intermittendly? Only needs it to fault a few times whilstdriving to get a fault logged.

[LC5F]

Thats how it should look, i'm guessing you're getting MIL on intermittendly? Only needs it to fault a few times whilstdriving to get a fault logged.

Thanks - that's good to know.
IKt first popped up 5 weeks ago and then this last week - but I really don't do much millage.
Will try the ald1717 suggestion of sensor spacer to see if that helps.

[pudding]

This all suggests it is a failed lambda - but as there is bank 1 sensor 1 & bank 1 sensor 2 how do I pin down what one is fubar?
How come VCDS doesn't provide any breadcrumbs to say what one is misbehaving?

It will tell you if it's B1 S1 or B1 S2 that's gone down    The breadcrumbs are Bank(s) and Sensor number(s).......because......Audi V8s with 2 banks and 4+ sensors.

What you can't know, however, is which cat it's moaning about since B1 S1 is upstream of both cats, and B1 S2 is downstream of both cats.  Or if you've bashed the pre-cat out, then the answer is easy. If B1 S1 goes down you'll know about it as it will run like crap.

If the pre-cat is removed, it puts more load on the secondary cat, and if the car is also remapped......it's just a matter of time.

[ald1717]

This all suggests it is a failed lambda - but as there is bank 1 sensor 1 & bank 1 sensor 2 how do I pin down what one is fubar?
How come VCDS doesn't provide any breadcrumbs to say what one is misbehaving?

It will tell you if it's B1 S1 or B1 S2 that's gone down    The breadcrumbs are Bank(s) and Sensor number(s).......because......Audi V8s with 2 banks and 4+ sensors.

What you can't know, however, is which cat it's moaning about since B1 S1 is upstream of both cats, and B1 S2 is downstream of both cats.  Or if you've bashed the pre-cat out, then the answer is easy. If B1 S1 goes down you'll know about it as it will run like crap.

If the pre-cat is removed, it puts more load on the secondary cat, and if the car is also remapped......it's just a matter of time.

think he said he has a magnaflow downpipe with sports cat, so im guessing the 02 sensor for emissions is throwing a wobbly

[GVK]

If the rear 02 sensor has a fault that would bring up a different code then the 420.

In the case of a 420 the rear sensior voltage should mirror/follow the front sensor.

if the cat is good the rear sensor voltage should be static and not wave about at all in ideal conditions.

[LC5F]

Yep - it's my Pipeworx 200 cel magnaflow sport cat down pipe with that gave excellent CO figures in late October MOT of:

Fast idle - 0.07% - Maximum of 0.20%
Natural idle - 0.04% - Maximum of 0.30%