Your VW Jetta is equipped with a computer controlled engine which delivers the combustion chambers precise amounts of fuel in order to combust as efficiently as possible. Any emissions component or sensor not functioning properly can cause an increase (or decrease) in fuel delivery, disturbing air/fuel mixture and resulting in incomplete combustion; hence the high Carbon Monoxide (CO).
CO is a by-product of incomplete combustion. Unspent fuel (due to incomplete combustion) is sent out the tailpipe and will contain partially burned fuel (CO) and un-burned fuel (HC).
In a "rich fuel" condition such as with your VW Jetta the diagnosis should include inspecting all possible mechanical faults which could cause excessive fuel to enter the combustion chambers. This includes inspecting your VW Jetta for high fuel pressure, air restrictions and/or leaking fuel injectors.
Your Volkswagen's fuel injection “feedback” system should be inspected as well. This includes ensuring the oxygen sensors are working properly, your Jetta's emissions control computer (ECC) is receiving the signals from the oxygen sensors, the computer is computing the data properly and sending the correct signals to the fuel injectors to either increase or decrease fuel delivery.
The additional sensors which should be inspected for proper operation (and which also play a significant role in determining your VW Jetta's engine air/fuel ratio) are the Engine Coolant Temperature (ECT) sensor and the Mass Air Flow (MAF) sensor.
If your VW's engine coolant temperature sensor is not operating properly (not sending the correct voltage signal to the computer, the ECC will not be able to properly calculate how much fuel to deliver to the combustion chambers. When the engine is cold the ECC is programmed to deliver a richer fuel mixture (more fuel). As the engine warms up, the ECC will lean out the mixture. Insuring the ECT is sending the correct voltage to the ECC is important for correct air/fuel ratio (optimum is 14:7).
