Ive owned my (2000) Mark 4 Golf GTi (67K) a couple of weeks, the other day the exhaust emission warning light lit up.

So i decided to invest in a VAG COM, ive done a scan and its brought up this result

16806 - Main Catalyst : bank 1 : Efficiency Below Threshold
P0422 - 35-00 - -

After doing some investigation the most likley cause is a lambda sensor. But which one is it likey to be the pre cat or after cat one ?

If it helps the car hasnt been idling very well and the MPG isnt great.

Any help would be appreciated.