1994 Toyota Previa 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.