Tips on How to Pass the California Smog Test

by Chris Stevenson
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The smog test, a requirement of California state law, checks that vehicles pass certain emission standards. Your engine cannot exceed standards for hydrocarbon (HC) and carbon monoxide (CO) emissions, since such chemicals can be harmful to the atmosphere. Preparing for a smog test requires a few systems checks and basic knowledge. If the vehicle owner knows what to expect and what to look for, he can pass the California smog test with relative ease.


Fresh gas burns much more effectively than gas that has sat in a vehicle over a prolonged period of time. Gas can varnish and accumulate moisture in an idle tank. A vehicle owner should purge his tank of any old gasoline, and then refill it with the highest octane gas available. The higher octane fuel will burn hotter and faster, causing a leaner combustion. The leaner combustion reduces HC and CO emissions.


Vehicle tires should be inflated to their maximum allowable limit, as dictated by the manufacturer's service recommendations. During a smog test, vehicles must be checked on a dynamometer to simulate low and midspeed driving conditions. Properly inflated tires cause less friction, allowing the engine to work easier to maintain rpm. An engine that runs easier burns fuel more efficiently.


A vehicle that has been properly tuned before a smog test will likely pass. Misfiring spark plugs, out-of-adjustment carburetors or clogged fuel injection, incorrect timing and other ignition maladies will show up instantly during the smog test. A proper tune-up that includes points, plugs, timing check, carburetor or fuel injection service, air cleaner and exhaust system inspection will aid in passing the smog test. The choke must be properly set. An oil and filter change will help seal the combustion chamber, preventing blow-by gases.

Check Engine Light

A "Check Engine" light on your instrument cluster denotes a system on your vehicle that requires attention or has failed. A smog technician cannot pass a vehicle that has a check engine light. A vehicle owner should take their car or truck to a certified dealer that has code scan equipment, which will pinpoint the system failure. The problem must be repaired, upon which the technician will reset the engine light.


A smog technician will not a pass a vehicle that has moderate to heavy fluid leaks, including oil, coolant, brake or transmission fluid. Such leaks can denote major engine problems, but also serve as a safety and liability factor for the smog station. A vehicle that overheats and leaks during the test will fail, so precautions should be made to fill the radiator and check the integrity of the radiator cap and all hose connections.

Vehicle Load

A vehicle loaded down with extra items or gear in the trunk or floorboard will cause extra load on the engine during the test. All such items should be removed, including bicycle racks and luggage carriers. Any extra load will cause engine bogging.

Operating Temperature

One of the most common failures during smog tests involves engines that have not reached normal operating temperature. The engine should be allowed to heat up thoroughly before the test. This does not mean merely idling, but a lengthy run on the freeway just prior to the smog test. Properly heated engines burn fuel much more efficiently than warm or cool engines. It also ensures that the choke will open fully.

Gas Cap

Filler gas caps should be properly tightened and sealed correctly. Many fuel tanks operate under a slight pressure. A loose gas cap can trip the vehicle computer and cause a check engine light.

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