See Figures 1 and 2
The emission control devices required in Chevrolet and GMC pick-ups are determined by weight classification. All trucks under 8,600 lbs. GVW are light duty emission models and use the same controls as can be found on cars. All four wheel drive vehicles and trucks over 8,600 lbs. GVW are considered heavy duty models and use fewer emission controls.
With emission level maintenance standards getting stricter on state and Federal levels, proper testing and service of each system becomes more important. Much confusion results from the variety and combinations of systems used in any year. Factors such as engine, build date, and geographic location are just some variables that determine what emission controls are installed on the truck. Not all of the systems described are in all vehicles, it's always best to consult the truck's vehicle emission label to determine what systems are installed on the truck your working on. The label also includes a vacuum schematic of the system on the truck which is very helpful when servicing the emission systems. If the emission label is missing or illegible a new one can be ordered at the dealer, but call the dealer ahead of time to find out what information is required to order the correct label.
One very important point to keep in mind when working on the emission system is to tag or identify ALL hoses and wires that are removed during service. Reinstalling wires and hoses incorrectly can cause disable emission systems, leading to excessive exhaust emissions and poor driveability. Attempting to reconnect the various vacuum hoses without a reference point can be extremely difficult.