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ASE P2 TEST PRIMER: Emissions Control Systems

Emissions are tightly regulated and must not exceed federal limits.



Emission control systems include the engine management system (PCM and sensors), Positive Crankcase Ventilation (PCV) system, Exhaust Gas Recirculation (EGR), the Evaporative Emission Control System (EVAP) and catalytic converter.

Emissions are tightly regulated and must not exceed federal limits. All 1996 and newer passenger cars and light trucks are equipped with an OnBoard Diagnostic (OBD II) system that monitors emissions compliance. If a fault occurs in any monitored system or component that might cause emissions to exceed the federal limit by 1.5 times, one or more Diagnostic Trouble Codes (DTCs) will be set and the Check Engine Light (also called Malfunction Indicator Lamp) will be illuminated to alert the driver. A scan tool can then be used to read the fault codes and to perform additional diagnostics. A vehicle will not pass an emissions test with fault codes or a Check Engine light on.


Common emission faults include failing oxygen sensors that upset the engine’s air/fuel mixture, engine misfires (which may be caused by ignition, fuel or compression problems), EVAP faults (a loose gas cap is a common one), EGR faults and low catalyst efficiency (which usually indicates a failing catalytic converter).

The PCV system recirculates crankcase blowby vapors inside the engine by rerouting them back into the intake manifold. This prolongs the life of the motor oil by preventing sludging and prevents blowby vapors from escaping into the atmosphere. The PCV valve is usually located in a valve cover, and is attached to the intake manifold by a hose. The recommended replacement interval is typically 50,000 miles. PCV valves have different flow characteristics, so the replacement valve must be the same as the original.


The EGR system reduces the formation of oxides of nitrogen (NOx) by cooling peak combustion temperatures. EGR also reduces the risk of engine-damaging detonation (spark knock). The EGR valve, which is mounted on the intake manifold, recirculates exhaust gas back into the intake manifold when the engine is under load. Older EGR valves are vacuum-operated, but most newer ones are electronic.
The EVAP system prevents the escape of fuel vapors from the fuel system and fuel tank. Fuel vapors are vented to a charcoal-filled storage canister, and are then vented into the engine through a purge valve to be reburned when the engine is running.


The catalytic converter reduces pollutants in the exhaust. The catalyst triggers chemical reactions that reduce unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). Catalyst efficiency may drop if the catalyst becomes contaminated by phosphorus (which may happen if the engine is burning oil), or by silicates if the engine has an internal coolant leak (leaky head gasket). The catalyst also may overheat and sustain damage if the engine is misfiring. Replacement converters must be the same type as the original, OBD II certified for 1996 and newer vehicles, and “CA-approved” for California vehicles.

Required to pass this section of the P2 test:
1. Identify major emission control systems and components.
2. Identify component function and common reasons for replacement.
3. Identify related items, including emission hoses.
4. Provide basic use, maintenance, installation and warranty information.

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