got an import impreza sti which failed its mot on emissions

at high idle

c/o was 2.5%
passed on hydrocarbons and lamda was .096.

at tickover c/o was 1.99%

the car had been decatted,so i have refitted a cat into the exhaust system and took it back for retest

at high idle

c/o is 1.99%
hydrocarbons still ok and lamda ok

at tickover
c/o is now 0.51%


he has still failed it as the c/o is too high.

was going to change the lamda sensor,or should i check anywhere else.

it is fairly heavy on fuel,could a remap sort it out?