CTS, classic this is.
A new one is about £10 from MATT LEWIS 01922 692424.
Put this in check the wiring to it carefully, the reset your CO to 2-2.5% CO, it will, then sail through.
The CTS or COOLANT TEMPERATURE SENSOR is the main corrective factor for the CO(mixture), ie it determines about 90% of it, with little trims from other inputs like air temp sensor.
tabetha