CTS sensor, check first it's plugged in.
Disconnect the ISCV and see what the idle does, set it to 850 rpm, with HOT engine and NO LOAD, ie lights etc off, check for any obvious obstructions like a tight throttle cable, or sticking flap etc at the same time.
You need to check the CO should be around 2 - 2.5 % CO.
It sounds like someone has been tinkering before you, a huge amount of people turn the CO screw to try to cover up for other faults like a duff CTS.
It would be a good idea if you do not know when it was changed to change the CTS, only 10 minutes and £10 or so.
I get mine from MATT LEWIS 01922 692424.
tabetha