The Smog Check: Three Different Tests Performed
Emissions Test
During the emissions test, the vehicle is driven on a dynameter (machine to allows the vehicle's wheels to actually spin) while the smog machine collects and analyzes emissions coming from the tailpipe. The three main emissions measured are HC (Hydrocarbon), CO (Carbon Monoxide), and NOx (Nitrogen Oxide). When the vehicle is not burning gas efficiently, it will produce emissions levels higher than that set by California.
The emissions test is usually the part of the Smog Check where most vehicles fail.
Perform routine tune ups by changing the motor oil, spark plugs, fuel filter, and air filter. Regular maintenance can greatly improve your vehicles emission output by cleaning out engine oil contamination high in hydrocarbons. Most important, doing so will increase the vehicle's ability to pass the smog check.
Read more about smog in the FAQ
Visual Test
The smog check technician will inspect various components of the engine to ensure that they are properly and securely connected. These components include:
- Underhood Emission Label
- Charcoal Canister (Evaporation Control System)
- Oxygen Sensor (O2 Sensor)
- Exhaust Gas Recirculation Valve (EGR)
- Positive Crankcase Ventilation Valve (PCV)
- Catalytic Converter
- Air Injection System
If possible, check the connections of these components at home prior to driving the smog check station. Verify that they are fully and firmly connected and not visually worn down.
Functional Test
The smog technician will test the following engine components to ensure that they are working properly.
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Check Engine Light
The smog check technician will look to see if the vehicle's check engine indicator or light is on. If it is on, the vehicle fails the Smog Check automatically. Please ensure that it is off before going to the smog check station. If it is on, visit a chain auto supply store to get information on why the check engine light is on. -
Engine Ignition Timing
The smog check technician will check whether the vehicle's ignition system is supplying correct electric current to the spark plugs and at the correct intervals. -
Gas Cap & Filler Neck
The vechicle's gas cap will be checked to ensure that it holds the correct pressure and inhibits the gas fumes from escaping into the atmosphere. If the gas cap is malfunctioning, you may purchase a new gas cap from the smog check station. The filler neck will also be examined to ensure that it accepts and connects tightly with the correct gas cap. -
Exhaust Gas Recirculation Valve
The Exhaust Gas Recirculation Valve works by allowing a small amount of exhaust gas to recirculate back into the combustion chamber when the engine is prone to producing nitrogen oxides. Since exhaust gas cannot be burned again, combustion temperatures are reduced and, thus, the production of NOx is decreased. Because the EGR performs this important emissions reducing function, it is essential to ensure that this component is working properly. The EGR value is easily replaced and most replacements are available at the local auto parts store. -
Low Fuel Pressure Evaporative Test (LPFET)
On December 1, 2007, California Bureau of Automative Repair added the EVAP Functional Test (LPFET) to test vehicles manufactured from 1976 to 1995 including all pre OBDII. The test is performed on the gas filler neck as well.
