Emission Control Systems: Description and Operation

PURPOSE
- The California Air Resources Board (CARB) began regulation of On Board Diagnostics (OBD) for vehicles sold in California beginning with the 1988 model year. The first phase, OBD I, required monitoring of the fuel metering system, Exhaust Gas Recirculation (EGR) System, and additional emission related components. The Malfunction Indicator Lamp (MIL) was required to light and alert the driver of the malfunction and the need for service of the emission control system. The MIL must be labeled "CHECK ENGINE" or "SERVICE ENGINE SOON". Associated with the MIL was a fault code or Diagnostic Trouble Code (DTC) identifying the specific area of the fault.

- The OBD system was proposed by CARB to improve air quality by identifying vehicles exceeding emission standards. Passage of the federal Clean Air Act Amendments in 1990 has also promoted the Environmental Protection Agency (EPA) To develop on board diagnostic requirements. CARB OBD II regulations will be followed until 1999 when the federal regulations will be used.

- The OBD II system meets government regulations by monitoring the emission control system. When a system or component exceeds emission thresholds or a component operates outside of tolerance, a DTC will be stored and the MIL will be illuminated.

- The emission system controls tailpipe emissions, hydrocarbons produced by evaporating fuel, and combustion gases that escape through the crankcase into the atmosphere. Effective control of vehicle emissions is necessary to reduce the automobile's impact on environmental pollution.

OPERATION
- Fault detection strategy and MIL operation are associated with trips and drive cycles. Each monitor has requirements for setting and clearing DTC's and for controlling the MIL. The diagnostic executive is the computer program in the EEC-V PCM that coordinates the OBD II self -monitoring system. This program controls all the monitors and intersections, DTC and MIL operation, Freeze Frame data and scan tool interface.

- Freeze Frame data describes stored engine conditions such as, state of the engine, state of fuel control, spark, rpm, load, and warm-up status at the point the first malfunction is detected. Previously stored conditions will be replaced only if a fuel or misfire malfunction is detected. This data is accessible with the scan tool to assist in repairing the vehicle. The emission system has components that are monitored and components that are not monitored.

MONITORED COMPONENTS
- Exhaust Gas Recirculation (EGR) System Monitor
The EGR System Monitor tests the integrity of the circuit, components and hoses that make up the EGR system and determines EGR flow rate errors.

- Heated Oxygen Sensor (HO2S) Monitor
The H02S System monitors upstream exhaust gases to fine tune the air/fuel ratio. The downstream H02S is used for catalyst testing.

- Catalyst Efficiency Monitor
Uses the downstream H02S to determine when a catalyst has fallen below the minimum level of effectiveness to control exhaust emissions.

- Misfire Detection Monitor
Detects fuel, ignition or mechanically induced misfires. The system uses the crankshaft position sensor to determine which cylinder has misfired.

- Fuel System Monitor
The fuel control system uses the adaptive fuel table to compensate for normal variability of the fuel system components caused by wear or aging.

- Comprehensive Component Monitor
The system detects malfunctions of any electronic powertrain component/system that provides input to any other OBD II monitor.

- Secondary Air Injection (AIR) System
This system monitors the AIR function. It controls the emissions during the first 20 to 120 seconds after the engine starts and for the catalyst light off to function. Air is forced into the exhaust manifold to oxidize the hydrocarbons and carbon monoxide created by running rich at start-up.

COMPONENTS NOT MONITORED
- Fill Pipe Restrictor
To prevent the larger diameter fuel nozzles used for leaded fuels from entering the fillpipe, a fillpipe restrictor is installed.

- Positive Crankcase Ventilation (PCV).
The PCV system prevents blow-by gases from entering the atmosphere by recirculating the gases back into the intake system.