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Smog Check Basics

What exactly is a smog inspection?

In short, a smog inspection involves experts checking your vehicle’s emissions. The exact means depends on what sort of car you have and how old it is. There are three primary components to a California smog test: the emissions inspection, the visual inspection, and the functional inspection.

What is the emissions inspection?

The emissions inspection is fairly straightforward. An exhaust gas analyzer is used to check your car’s exhaust for hydrocarbons, carbon monoxides, and nitrogen oxides. The more they find, they worse it will look.

What is the visual inspection?

The visual inspection is a little more involved. The station will check your vehicle and make sure it has all the proper components, as well as that they are set up correctly. To start with, they’ll check for the:

  • Air Filter and Housing

  • Air Injection System

  • Camshaft Position Sensor

  • Carburetor Pre-Heater Tube

  • Catalytic Converter

  • Charcoal Canister

  • Crankshaft Position Sensor

  • Engine Coolant Temp Sensor

  • Exhaust Gas Recirculation Valve

  • Intake Air Temperature Sensor

  • Manifold Absolute Pressure

  • Oxygen Sensor

  • Positive Crankcase Ventilation

  • Throttle Position Sensor

  • Underhood Emission Label

On top of that, they will also take a look at whether your car produces an excess of smoke when running. It’s important to note that they don’t just look for black smoke, but white smoke as well.

What is the functional inspection?

In the functional step of the inspection, they will go through five steps.

First, they’ll check the engine ignition timing. Simply put, they’ll look at whether the device is set to be within three degrees of the manufacturer’s required setting. If it’s off by more than that in either direction, it can dramatically increase hydrocarbon emissions from your car.

Second, they’ll take a look at the check engine light. If it’s on, your car fails automatically. However, they don’t just check whether the light shows when the car turns on. They will also run several tests involving whether the light turns on under certain conditions.

Third, they’ll inspect the gas cap and filler neck. What they’re looking for is actually pretty specific and it’s unlikely to fail this stage unless you’ve made certain modifications to your car. They’re looking for whether the filter neck has been modified to accept leaded gas, so if you haven’t done that and if you didn’t buy the car from someone who did that, you should be fine.

Fourth, they’ll take a look at the exhaust gas recirculation valve. Not every type of car is equipped with an exhaust gas recirculation valve, but for those that are, they want to make sure yours isn’t collecting excessive carbon. A regular cleaning can help prevent that.

Fifth, they perform the low pressure fuel evaporative test. A pressure line is hooked up to your car’s gas filler neck, then turned on and run for two minutes. If too much pressure is lost, then that indicates a leak in your system and will lead to a failure on the test.

They won't necessarily go through every step exhaustively for every type of car, so you should check the specifications for your model.


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