If it needs a "blue sensor" then put one in first.
I assume from this it is the CTS, coolant temperature sender for the ecu to which you are reffering ?, if this is out the CO will be all over the place, as this is the MAIN determining factor for CO, others just trim a bit, so put a sensor in, check plug gaps then take to get at least the CO set to around 2.5% CO, and see where you go from there.
tabetha