You asked how the emission inspection program identifies possible causes of test failures and how accurate these diagnoses are.

After testing, the technician gives the motorist a Vehicle Inspection Report (VIR), identifying the pollutants emitted by the vehicle in comparison to the allowable standards. In the case of a failed test, the VIR will indicate the specific pollutant that exceeded these standards and the possible causes for this failure.

In the case of newer vehicles (1996 and later model years) the report is based on data from the vehicle’s on-board diagnosis (OBD) system. This system stores information regarding engine performance, including information about the vehicle’s emissions control systems. The OBD system is designed to identify deterioration of the catalytic converter or oxygen sensors, misfires, and other problems that can cause excessive emissions. When a vehicle’s sensors detect a problem, the OBD system sets a fault code and turns on the “check engine” light on the dashboard. The codes are stored in the vehicle's on-board computer for access by technicians, to facilitate proper diagnosis and repair.

In addition to the OBD system, there are 10 different vehicle systems that may affect the outcome of an emissions test. These are the air induction, fuel metering, ignition, exhaust gas recirculation, positive crankcase ventilation (PCV), fuel evaporative control, secondary air injection, exhaust, and engine emission control systems, and related parts such as hoses and gaskets. Each system has multiple parts that may be the cause of a test failure. For example, the fuel evaporative control system includes the fuel tank, fuel lines, and gas cap.

Because of the complexity of these systems, the diagnosis of the problem contained in the VIR is not always accurate. Repair technicians use the information in the VIR and the vehicle manufacturer’s manual to identify and repair the problem causing the excessive emissions, but a fault code can have several possible causes. In addition, problems can cascade, complicating diagnoses. For example, if a misfire is left uncorrected, too much fuel may be dumped into the catalytic converter causing it to fail. As a result, the excessive emissions may be due to a problem other than the one identified in the diagnosis.

Further information about test procedures is available on the program’s Website, http: //www. ctemissions. com/fail. html.

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