The measuring of the atmospheric oxygen as compared to the exhaust gases is accomplished by the GMC oxygen sensor. It is located on the exhaust pipe just beyond the exhaust manifold on either side of your catalytic converter. Unlike the name suggests the oxygen sensor does not register the amount of oxygen in the exhaust gases, but instead determines the ratio of the oxygen in the exhaust gases as compared to what is currently present in the atmosphere around the GMC truck. This is the information that is feed to the onboard computer. The GMC oxygen sensor is one of the many sending units that provide the data for the onboard computer to make the correct determination in just what the air to fuel ratio being supplied to the combustion chambers should be. The reason for this oxygen ratio to be known is so the different altitudes the GMC truck might encounter can be used in this determination. The higher in elevation the truck travels, the less oxygen there is in the atmosphere. As with all electronic components, the oxygen sensor will become faulty at some point in time when the truck is running. This fault will be indicated by the check engine light becoming illuminated on the dash. A specific code for a faulty oxygen sensor will then be stored in the onboard computer that can be retrieved. A list of just what all codes refer to can be obtained in a service manual for your GMC truck. Until the faulty oxygen sensor is replaced, the air to fuel ratio being supplied to the combustion chambers will not be at its optimum levels. This will adversely affect the performance and mileage of your vehicle until this situation is correctly addressed.