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Why Emission Readings Vary From Test To Test
If your vehicle has failed an emission inspection and then passed without any repair work being done, or if it has failed again in a different area or with much different readings, you have witnessed the effects of variability. Although it is common to believe that variation in emission readings is the fault of the test equipment, it is the vehicle that is varying, not the exhaust gas analyzers. No vehicle will produce exactly the same emission readings on back-to-back tests. However, if a vehicle's engine is in good mechanical condition, at its normal operating temperature, and has all of its original emission control equipment in place and operating properly, it will consistently produce readings within a narrow range well under the "maximum allowable" limits.

What causes variability?
The composition of the exhaust gas coming out of the tailpipe depends on many factors. In a six-cylinder engine running at normal cruising speed, air and fuel are being mixed and burned in over a hundred separate combustion events every second! The products of these combustion events then typically go through a catalytic converter that further acts on the hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx) before they come out the tailpipe. With all of the chemical reactions taking place inside the engine and downstream of the engine in the catalytic converter, it is little wonder that emission readings tend to vary. Computers that determine the right fuel mixture and ignition timing for a wide range of engine-operating modes control modern engines. To do this, the computer needs to collect information from a number of sensors telling it the engine speed, load, throttle position, air flow and exhaust gas oxygen content.

Defective sensors, wiring faults or corroded connectors can produce intermittent false inputs to the computers. When such faults are present, emissions are high; when they're not, everything may look perfect.

Is the emission reading accurate?
The analyzers used to test vehicles are both accurate and reliable. Emission analyzers in the lanes are calibrated every business day and the zero level is checked and re-sets between each test. Audits are performed on the analyzer using federal government certified blends of HC, CO, NOx and C02, in order to ensure that the analyzer will accurately read the concentration of any gas presented to it. The dynamometer used for the driving test portion of the inspection is calibrated monthly to ensure that the desired test speed and load is delivered every time. Every precaution is taken to ensure that the equipment is ready and able to perform the test. The one big variable that we have no control over, however, is your vehicle. You can be sure though, that the reading you get printed on your inspection form accurately reflects what your vehicle was performing like at that moment in time. If it gets different readings another time or at another place, it's the vehicle that has changed.

What do I do if my vehicle has variable readings?
If your vehicle is failing the test with widely varying results, consult a recognized repair technician. The problem that is causing the failure is also causing the variability. If your vehicle passed without repairs, it could be that the problem that resulted in the failure is intermittent in nature or that conditions immediately prior to the test helped to mask the problem sufficiently to obtain a marginally passing result. In either case, you should still have the vehicle checked by a qualified technician. Because intermittent problems may not be present when the vehicle goes to the repair shop, the diagnostic process may take more time. However, a thorough check of all systems will eventually reveal the source of the problem.