Anyone know what engine mgmt light code P0420 means?

Car 2007 STi hawkeye
Recently mods (past week):
Walbro 255 fuel pump
H&S decat downpipe
Last cat between downpipe and centre pipe left in place
Mods done by Subaru during 4th yr service
Mods done in prep for a remap by JGM scheduled for next Tuesday

Tonight while cruising along at 40 mph steady, engine mgmt light comes on.
Stays on and won't reset after 3 restarts
Queried with Vi monitor which report code "P0420 Unknown"
Light reset with Vi monitor

Anyone know what code P0420 means?

 

Catalyst below threshold p0420

recent decat will trigger the fault code, when mapped the code will be erased

 

You're a gem mate. Thanks a mil.

 

also turning ignition on and off 5 times will turn the light out for a short period if it bothers you

 

Ah - I thought it was 3. I have a Vi monitor which allowed me to reset it. Cheers mate.