# Check Engine light code P0421



## Mawman82 (Sep 10, 2009)

My Check engine light came on and said Emissions Workshop, and when i went to get the code it came up with "Warm Up Catalyst Efficiency Below Threshold (Bank 1)". Any ideas on what it might be? I am thinking that it is just an oxygen sensor, but i want to make sure its not the entire cat thats the problem..


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## tryin2vw (Dec 30, 2006)

*Re: Check Engine light code P0421 (Mawman82)*

Also check your Purge valve (black round thing on the air cleaner) for proper operation. If it is stuck open, a large slug of gas vapors will hit the cat at start-up, another cause of the code you list. 
The cat and O2 sensor operate ok when the car is warmed up, since there is no code.


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## dumped_gti (Dec 16, 2003)

*Re: Check Engine light code P0421 (tryin2vw)*

NO NO NO! not an evap purge valve.. You would for sure have an "evap incorrect flow detected" fault if it were stuck.
You need a new catalytic converter (passenger side). you should also replace the front and rear oxygen sensor at the same time. And don't put on a cheap cat. get a quality OEM part. I have seen this 100 + times http://****************.com/smile/emthup.gif 


_Modified by dumped_gti at 8:31 AM 9-13-2009_


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## tryin2vw (Dec 30, 2006)

*Re: Check Engine light code P0421 (Mawman82)*

Check the cheap stuff before going to the cat. Check your O2 sensors using a VAGCOM, check the Purge valve. The cat will cost you way more than these, so check them first. Note that I did not say replace, but verify they are OK.
Doug


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