2004 GMC Envoy XL Emission System Diagnosis & Testing Cost

For several decades now, manufacturers around the world have been required to make advances on reducing pollution caused by the overwhelming acceleration in the personal and business use of automobiles. This had led to the development of several excellent subsystems that all work together to capture production of harmful gases and chemicals of various forms before they are sent directly into the air we breathe. These subsystems have many different functions, but they all come together to accomplish that single task as the emission control system.

Preventing harmful emissions starts where it is least expected, the fuel tank. Evaporation of fuel for internal combustion engines is extremely damaging to the environment, so when fueling a vehicle, a system is used on the fuel pump to trap fuel vapors before they travel outside of the system. Furthermore, once fuel is inside the fuel tank, evaporation will cause the fuel vapors to be trapped in a charcoal canister, and valves will be opened to suck those vapors into the engine. This, again, prevents those vapors from moving from the fuel tank into breathable air. Next, the engine management system controls the operation of the engine to use as little fuel as possible, maximize the efficiency of fuel used, and return unused fuel to the fuel tank. This is all done by precisely managing every aspect of how the engine operates. Once fuel is burned, it is either flowed back through the engine to be reburned, or it flows out of the exhaust system. To prevent formation of other harmful chemicals, as well as dumping raw, unburned fuel into the air and onto the ground, the catalytic converter completes the burning process to clean the air moving to the atmosphere.

Emission system symptoms have one commonality that will nearly always be present. The check engine light will illuminate, and codes will be stored in the powertrain control module (PCM) alerting the driver that an emission issue is present. Besides this small commonality, emissions system failures have limitless symptoms that can arise because the emissions system is tied into everything the engine and transmission does, as well as all of their accessory symptoms. That means the vehicle could display symptoms like jerking, hesitation, failing to shift properly, failing to start, stalling, rough idle, and the list could go on.

Emission system components should be repaired immediately to prevent excessive pollution from your vehicle, but more than that, to return the vehicle to proper operation. In many causes, the check engine light may be on, but no other symptoms are present. For these situations, the driver has a little time before the repair must be done. In general, the check engine light should not be ignored.

Again, since the emission system encompasses so many aspects of the drivetrain, the chance of an emission related component to fail is high. Most vehicles will have a repair to one or more emissions components during their useful service life. Some systems, like the EVAP and EGR systems, seem to have higher failure rates than others.

Diagnosis of the emissions system will nearly always begin by connecting a professional grade diagnostic scan tool to the vehicle. That is because the OBD-II trouble codes stored in the ECU will need to be accessed, read, and evaluated. This is normally the first step towards diagnosis. The subsystem registering the fault will then need to be inspected, tested, and the faulty component will be identified and proven faulty before recommending a repair.

RepairPal recommends the proper maintenance and repair of all emissions systems for every vehicle. Although many states do not inspect the operation of the emission systems, these systems are put in place to protect the environment to the best of our ability. Depending on location, removal or tampering with the emissions system may be illegal, and, at the very least, the vehicle will likely lose a considerable amount of fuel efficiency.

The check engine light warns drivers about issues related to emissions control systems. You may notice that many things, like spark plug replacement, may cause the check engine light to turn off, and wonder why that would be an emission related component. This is because the efficiency of the engine is controlled to a high degree to reduce harmful emissions, and a bad spark plug will cause elevated harmful emissions. This is why so many subsystems play a role in emission control.

Diagnosing the emission system is likely more difficult than many DIY mechanics will reasonably choose to take on. That being said, there are many components that can be easily diagnosed and repaired, but, in general, check engine light diagnosis and emission system diagnosis should be left to a professional if the cause and remedy are not readily identifiable.