The CTS ideally should be chnaged every few years, as it is so important and is the major governing factor for CO.
They cannot be cleaned as they are a sealed unit, the fact that it is ok when up to temp suggests CO is a bit weak when cold, this won't be due to a duff iscv, as all this does is raise the revs, it has no control over mixture.
I would put in a new CTS, then adjust to around 2% -2.5% CO, or what it likes best, if it is chipped seek the advice of the author of the chip.
tabetha