SmogTips.com lists a few things that you should check:
1. Air Filter
A dirty air filter will disturb the normal air / fuel ration and result in less than optimal fuel efficiency, consequently resulting in higher CO emissions.
2. O2 Sensors
The computers in your car are hooked up to the Oxygen Sensors. The computer adjusts the amount of fuel going into the engine based on the amount of oxygen in the air registered by the oxygen sensors. If the O2 sensors are giving a faulty reading, the computer will deliver less than the optimal air / fuel ratio and you could be putting out more CO.
3. MAP Sensor
This is another component that sends information to the central computer of the car. It tells the computer how strong a vacuum is being generated during the engine's intake stroke. If it is defective, it will not report the information correctly, and the fuel / air ratio will be messed up.
4. Throttle Position Sensor
If the throttle plate (usually connected to the gas pedal via a cable) is being detected in the wrong position, the engine will send more or less gas into the engine than it ought to, resulting in a less than optimal fuel / air ratio.
5. Engine Coolant Temperature Sensor
When the engine is colder, it requires more fuel. Hence, if the engine "believes" the coolant to be a different temperature than it is, it will put in more or less fuel than required, resulting in a faulty air / fuel ratio and potentially more CO emissions.
Hope this helps!