hmmm i was going to say either the act or cts sensor when were they new as you could have buggerd one?

the sensors are basically big resistors, when the car is warm they give little resistance so the ecu then cuts the fuel back.

there is a way that i used to test them but not sure if its a good idea or not as it tended to blow the 30amp fuse found near the ecu

it will tell ya if the sensor is working though

warm the car up and unplug the sensor, on the plug, bridge the 2 contacts with a bit of wire. this will give the ecu the same low resistance signal that it would normally get if the sensor is ok

if when you do this the co comes down to normal then you have a bad sensor

or like jim says it could be a split vac pipe