EML catalyst system bank 1 code

[markos]

so i fitted this exhaust a couple of months ago and everything was running sweet (and still is) apart from a couple of what i thought were misfires

http://www.powervalve.co.uk/products/vw/golf/mk5/golf-mk5-gti-edition-30-2004-2007/vw119-downpipe-with-200-cell-motorsport-cat-and-link-pipe-to-fit-oem-system-with-heat-wrap-system-options-sports-system.html

then about 2 weeks ago i fitted the gfb dv+ again with much improved performance.

but, this morning i had an engine management light flick on. just had it diagnosed and this came up:

01056 Catalyst System Bank 1 Efficiency Below Threshold P0420 Lower Limit Reached

any ideas as to what this could be? faulty sensor maybe? the "misfires" didn't come up though which was interesting

[markos]

update - spoken with the guys at powervalve and it seems i need to fit a lambda boss extension as the ecu is freaking out about the extra flow coming through the sports cat.

anyone done this and know if it's an easy job?