Hi all, pulled on the M1 today, zipped on in 4th then 5th, started cruising then the management light came on and the car ran a bit rougher and the fuel gauge was falling quicker than usual. i drove the car steady till i got back home and the code is P420, Catalyst Efficiency below threshold ( bank 1 ).
My car has had the sti up pipe and EGT bypass fitted for over 6 months with no probs.
What do i need to do, fix, change etc to remove this code ???

 

Needs the code mapping out