I HAVE A PROBLEM EMISSIONS!!!!! PO420. ADVISE PLEASE.....

[cominatcha]

I HAVE A AUDI A4 AVANT 190 BEX 04 PLATE.............LAST YEAR HAD LIGHT ON PULLED THE CODE,PO420. TOLD TO REPLACE LAMBA REAR. DID THAT STILL ON AFTER 100 MILES. TOLD TO REPLACE CAT DID THAT AGAIN ON AFTER 100 MILES. TOLD TO CHANGE FRONT LAMBA DID THAT ON AGAIN AFTER 100 MILES. PUT IN FULL TANK OF SUPER DROVE 400 MILES ALL OK THEN LIGHT BACK ON. PUT LIGHT OUT PUT THROUGH MOT ALL OK. TWELVE MONTHS ON LIGHT HAS COME ON ABOUT THREE TIMES. OK PUT THE IN FOR A MOT GUESS WHAT FAILED ON HIGH EMISSIONS OF A CODE PULLED PO420..............I HAVE LOOKED FOR AIR AND VAC LEAKS AT THIS TIME NONE..................ANY ONE HAD THIS OR ANY ONE SHED ANY LIGHT ON THIS... THANKS NEIL

[Pabs]

Does it have a lumpy or rough idle?? If it does check out the crankcase breather valve there is a thread about it

Dodgy Idling - 1.8T

Crankcase Breather Valve Location

[quattrojames]

From Ross-Tech

16804/P0420/001056 - Ross-Tech Wiki

Also they link to this for the 1.8T

VWVortex.com - Aftermarket DP/Cat -DTC - Cat efficiency below threshold - Who has this DTC and wich DP/Cat

[cominatcha]

today i noticed black soot around the rear lamba sensor as mechanic left it held in by only two complete turns...... i have cleaned and fitted tight. would this give a poor emission? thanks for my replys neil