What Are the Causes of Smog Test Failure?

by Erin Dorrance

State and federal governments have placed limits on harmful gasses emitted by vehicles. An emissions test, also referred to as a smog test, will measure the hydrocarbons, carbon monoxide, carbon dioxide and oxides of nitrogen gas levels. There are several common causes that lead to a vehicle failing a smog test.

Check Engine Light

If your check engine light or malfunction indicator lamp is illuminated, it will result in an automatic smog test failure. If your light is on, take your vehicle to an auto repair shop so they can diagnose your problem.

Old Oil

Your vehicle's engine breathes in fumes from oil pans and other auto components. These fumes make their way through the combustion and contaminate the engine oil. Changing your oil regularly will eliminate these fumes and allows your vehicle to pass the inspection.

Low Tire Pressure

Inflate your tires prior to a smog inspection. During the smog test, a technician may need to drive your vehicle on a dynamometer. Inflated tires improve the stability and accuracy of this part of the exam. Furthermore, the overall emissions of your vehicle are improved by letting the engine maintain a steady load.

Diagnose Gas Failure

Read the smog failure report to evaluate which gas caused the failure. An HC failure results from incomplete combustion. If you had an HC failure, the most common causes are worn spark plugs, defective spark plug wires, a worn distributor cap or rotor, improper ignition timing, vacuum leaks or an engine mechanical failure. A CO failure stems from an excessive rich air fuel mixture and is typically caused by an inoperative air injection system, a defective oxygen sensor, a leak or defective fuel injector, a restricted air filter, a vacuum leak from an improper manifold absolute pressure sensor operation, a defective air mass or air flow sensor, a defective catalytic converter, a defective thermostat, oil-contaminated fuel or a malfunctioning fuel evaporation system or purge valve. Lastly, a NOx failure comes from combustion temperatures that are running too high. The most common causes of a NOx failure is a cooling system malfunction, an over-advanced ignition timing, an excessive lean air fuel mixture or an inoperative gas recirculation system.

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