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ASE Tesp Prep: Emission Control System

There will be three questions on the ASE P2 test that deal with the emission control system, components and subsystems.


All 1996-and-newer vehicles have Onboard Diagnostics II (OBD II) as part of their programming to detect and identify emission faults. OBD II monitors ignition misfires, the efficiency of the catalytic converter, the operation of the engine’s sensors and feedback fuel control loop and many other functions.


One way the engine control system minimizes pollution is by carefully controlling the air/fuel mixture. The engine computer does this by monitoring the amount of unburned oxygen (O2) in the exhaust with an oxygen sensor mounted in the exhaust manifold. A voltage signal produced by the O2 sensor tells the computer if the mixture is rich or lean. The computer then compensates by decreasing or increasing the on-time of the injectors.

In addition to the Powertrain Control Module (PCM), key engine sensors and various subsystems are used to control specific pollutants. These include the Positive Crankcase Ventilation (PCV) system, the Exhaust Gas Recirculation (EGR) system, the Evaporative Emission Control System (EVAP), the Air Injection Reaction (AIR) system and the catalytic converter.


The PCV system recirculates blowby vapors from the crankcase back into the intake manifold so the vapors can be reburned inside the engine. This prevents the escape of blowby vapors into the atmosphere, and also helps remove moisture from the crankcase to prolong the life of the oil and prevent the formation of engine-damaging sludge. The PCV valve is usually located in a valve cover, and is attached to the intake manifold by a hose.

The EGR system reduces the formation of oxides of nitrogen (NOx) in the engine by reducing peak combustion temperatures when the engine is under load. When the engine is under load, intake vacuum drops. This causes the EGR valve on the intake manifold to open a port between the intake and exhaust manifolds. Loss of EGR can result in increased emissions and engine-damaging detonation. The EGR valve is calibrated to the engine application, so a replacement valve must have the same flow characteristics. Some replacement EGR valves have various adapters to modify the flow rate.


The EVAP system controls evaporative emissions from the fuel system and fuel tank. Fuel vapors are trapped in a charcoal-filled canister, then vented into the engine through a purge valve to be reburned. A leaky fuel filler cap can allow fuel vapors to escape into the atmosphere. Replacement gas caps must be the same type as the original (caps for some older vehicles may be vented, but newer ones are sealed). Fuel vapor leaks are monitored by the OBD II system.

The Air Injection Reaction (AIR) system on older vehicles uses an air pump to pump extra oxygen into the exhaust system to reduce pollution. A diverter valve assembly on the air pump controls the flow of air into the exhaust manifold. On some vehicles, air is also routed to the catalytic converter. An anti-backfire valve prevents hot exhaust gases from flowing backwards into the system and damaging the diverter valve or air pump. Problems in this system can cause an increase in emissions.


The catalytic converter is an “afterburner” in the exhaust system that helps reburn unburned pollutants. The catalyst inside the converter can be contaminated by lead (leaded gasoline), silicon (coolant leaks) or phosphorus (burning oil), reducing its efficiency. On 1996-and-newer vehicles with OBD II, a second oxygen sensor is mounted behind the converter to monitor its operation. If converter efficiency drops below a certain level, it will turn on the “Check Engine” light.

Converters can also be damaged by overheating if an engine is misfiring, leaking compression or burning oil. If the converter gets too hot, the ceramic honeycomb inside may melt causing an obstruction in the exhaust. A plugged converter will create backpressure that reduces engine performance and may even cause the engine to stall. There is no way to clean a plugged or contaminated converter, so replacement is the only option. Replacement converters must be the same as the original.

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