1991 GMC G3500 Emission Control System Inspect Cost

The emission control system of the vehicle is a group of components from multiple systems that work together to reduce the harmful environmental effects of using internal combustion engines. This system focuses on reducing emissions by increasing the efficiency of the engine, reducing evaporation of fuel, and cleaning the bulk of pollutants from exhaust gases. In fact, most electronic components on an engine relate directly to emissions. When the check engine light comes on, it signals an emission related failure, and related components of the emissions system will be inspected while diagnosing the emissions system failure.

The emission control system begins with the evaporative emission control system (EVAP). This systems prevents fuel vapors from evaporation from escaping from the vehicle. Since fuel evaporative emissions are among the most damaging to our environment, this is perhaps one of the more important systems. As fuel evaporates, it is held in a container until it is released into the engine to be used as fuel. Next, electronic ignition systems have increased the efficiency of engines by regulating fuel and air ignition according to many variables. Once burned, air and fuel is released into the exhaust system, but when possible, unburned air and fuel will be cycled through the exhaust gas recirculation system, as the name implies, to be recirculated through the engine and reused. Finally, the exhaust system uses oxygen sensors to prevent a large amount of fuel from passing through the engine unburned, and catalytic converters to reduce the release of specific chemical byproducts of combustion. There are several other components at work, and several other advances in technology that have led to making our combustion engines more efficient. It should be understood that the main goal of the emissions system is primarily focused on increasing engine efficiency to prevent the emission of pollutants.

Any time there is an emissions fault the check engine light will be displayed. The purpose of the check engine light is to inform the driver that an emission related fault has been found, and that there are on-board diagnostic (OBD) trouble codes stored in the powertrain or engine control module. Additionally, since the emissions systems are so intertwined into engine control and transmission control systems, symptoms may include nearly any sort of drivability concerns. This may include harsh shifting, failure to shift, hesitation on acceleration, jerking, engine failure to start or run, loss of power, or any number of other drivability issues.

Emissions system components in many states will require repair before the vehicle can legally pass inspection or operate on public roads. In other states, the vehicle may only lose fuel mileage, operate poorly, or lose power. In many instances, emission control components may cause a vehicle to become impossible to drive, unpleasant, or damaged. If the issue affects drivability or safety, the vehicle should be diagnosed as soon as possible.

Emissions control system components are exposed to harsh environments due to the nature of their operation. This means they will encounter raw fuels, extreme heat, high pressures, and carbon deposition. Due to these conditions, many emissions components will not last the lifetime of the vehicle. Catalytic converters, oxygen sensors, and EVAP system purge valves are the most likely to fail, and many of these will need to be replaced at or over 100,000 miles or so. There is no definite timeline for replacement, and the failure rate of the catalytic converter and oxygen sensors are directly related to how well the engine is maintained. The EVAP purge valve service life can be extended by avoiding overfilling the fuel tank with fuel.

Emission control systems are often diagnosed through the use of a scan tool which can command the operation of individual EVAP components. Diagnosing in this manner allows the technician to confirm the operation of several components within minutes. Once a failed component is identified, it will be replaced, however, the technician will likely have to conduct further diagnosis when dealing other portions of the emissions system. When the issue is related to the engine control system, diagnostics will take one of many paths, and the technician will need to identify a diagnostic method which is most likely to identify the problem in the most logical manner.

Correcting emission system issues may require a variety of procedures, but most repairs will require the diagnosis to be confirmed after the faulty component is removed in order to further prove the diagnosis. Often it is necessary to remove corrosion and dirt from electrical connectors in order to establish a good connection, and testing once the repair is complete is mandatory. Also, a faulty part may be found dirty instead of failed, and sometimes the repair can simply be to clean the dirty part. Significant disassembly of many components may be required in order to facilitate repairs to emissions system components integrated into the engine control system.

We recommend having emission system issues corrected immediately. Since a great portion of the engine control system, exhaust system and fuel system is dedicated to emission prevention or mitigation, these systems are sensitive to high emissions or inefficient operating conditions. Having these problems addressed sooner than later will likely reduce the chances of further damage.

Many states require that vehicles are tested for emissions compliance at the federal or state level. When vehicles are tested electronically, there are monitors that must be ready for testing. These monitors require drive cycles to be run in order for the vehicle to self-test and establish an internal pass or fail. These drive cycle tests often replicate how the manufacturer expects a vehicle to be driven, so many emissions testing facilities may recommend driving the vehicle in certain manners to ready emissions system monitors.

Repairing many emissions components are simple, fast, and relatively inexpensive, however, there are many components and systems involved with the emissions system that may require more expertise. In most cases, the diagnostic trouble codes will lead a repair in the right direction, but these codes only show failures, not failed components. If there are issues in understanding the trouble codes, tracking the cause of the failure, and testing emissions systems and computerized components, this should be left to a professional technician.