How Long Does a Smog Test Take?

Since the 1977 Clean Air Act was amended to require states to have a vehicle inspection, smog checks have helped improve the air we breathe. Emissions standards have reduced air pollution from vehicles that create unhealthy smog. Today’s cars are 98-99% cleaner for most tailpipe pollutants compared to vehicles of the 1960s.

If you live in a state that requires a smog check, it’s reasonable to ask, “How long does a smog check take?” We’ll answer those questions as well as detail what’s involved in a smog check.

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How long does a smog check take?

Once you are next in line, the process should take around 20-30 minutes. A couple of things will determine your time at the emissions control inspection location:

1. How many people are waiting? Because emissions inspections are necessary for tag renewals, many vehicle owners do them at the same time as you. For example, vehicles driven in California are required to have an emissions test every two years. In 2015, California tested more than 11 million vehicles. Of that number, more than 10% of vehicles do not pass.
2. You may not pass. If you have a 2-year-old vehicle, unless it’s damaged or tampered with, you’ll most likely pass the emissions test and be on your merry way. However, with older automobiles, parts begin to deteriorate, emission equipment begins to fail and general neglect of service will make a car prone to fail emissions testing. You’ll need to have the vehicle repaired and then returned for additional testing.

What is a smog check?

A smog check is a physical, computerized instrument inspection covering the vehicle’s emissions control and exhaust systems. The mechanic or inspector will visually confirm all of the emissions components are in place and functioning. Using test instruments and a state-certified computer diagnostics system, the inspector will plug into your onboard computer diagnostics system and monitor the emissions level of your exhaust pipe.

Generally, smog tests consist of testing for acceptable levels of these pollutants:

1. Carbon dioxide
2. Hydrocarbons
3. Carbon monoxide
4. Oxides of nitrogen
5. Plus, other emissions that come from your vehicle’s tailpipe.

Various inspections can check your vehicle’s emissions, but the following are the most common types.

On-Board Diagnostic (OBD) inspection

For the model year 1996 and later vehicles, the OBD inspection is the most conventional test. The OBD inspection reads data from your vehicle’s computer system to verify that systems are functioning correctly. If the engine is having a problem in any area, the ODB diagnostics test will report it with a verifiable fault code.

Acceleration Simulation Mode (ASM) test

OBD systems were not required in automobiles prior to 1995. Vehicles without the OBD system are given an ASM test using a dynamometer and a sensor for the tailpipe. The dynamometer is a treadmill for your car that is run by a computer and simulates driving conditions while the sensor sniffs your tailpipe emissions for pollutant levels.

Two-Speed Idle (TSI) test

A TSI test is reserved for cars built between 1976 and 1995. These vehicles have adjustable timing and idling. The test tracks exhaust emissions when the engine is idling at a high and a low speed.

If you’re from a state such as Montana and moving to California, you may wonder why California requires these tests and Montana doesn’t. The federal Clean Air Act specifies the maximum allowable levels of pollutants in the air for chemicals and elements like carbon monoxide, lead, ozone and sulfur dioxide. If a state exceeds those levels, federal law requires a plan to lower the pollutants.

That plan includes a smog check as well as vehicle emissions requirements. Although emissions levels and tests are also applied to companies that manufacture goods, states with vehicle smog checks believe vehicles are an additional contributor to pollution and require testing. The decision is always up to the state government.

Which states require a smog check?

Smog tests are becoming more widespread, and the majority of states require testing. Currently, you’ll find smog tests in 33 states, from Arizona to Wisconsin. Most states will have testing in heavily populated metropolitan areas, though locations may vary.

Why did I fail my smog check?

There are numerous reasons your vehicle may fail a smog check. Here are a few common problems.

Improperly sealed gas cap

A gas tank is part of the emissions control system. It seals off any evaporation or escape of fuel vapors into the air. An old gas cap can have a dried-out rubber seal that doesn’t seal off fuel vapors or tell the computer there’s a leak in the emissions control system. This is often a reason for a Check Engine Light warning. It’s an easy fix that requires purchasing a new gas cap at a local auto parts store.

Evaporative emission control system (EVAP)

Working in tandem with the gas cap, the EVAP is supposed to prevent raw gasoline vapors from getting into the atmosphere. An EVAP problem could be nothing—or it could be a significant warning sign. A mechanic could run diagnostics on your vehicle to find out what the issue is.

Ignition system defect

This can be anything from defective or old spark plugs or spark plug wires or other vehicle ignition system parts. If your vehicle ignition system fails, it will not burn fuel efficiently and will release hydrocarbons and fuel into the air.

Dirty air filter

Your engine runs most efficiently by combining a specific amount of air and a precisely measured amount of fuel. If your air filter hasn’t been changed, it could be clogged with debris and dirt, restricting the engine’s air intake. This is an easy and inexpensive replacement item.

Dirty oil

Dirty oil is high in hydrocarbons—pollutants that trigger a smog test failure as they circulate through your engine. Dirty oil releases fumes that end up in the exhaust and raise emitted hydrocarbon levels. The best thing is that changing the oil is a simple and inexpensive procedure. Clean oil in the engine helps provide clean exhaust. It’s also an excellent time to have the technician check for any other possible problems that will cause an emissions test failure.

Catalytic converter

Your catalytic converter may still be working, but it’s no longer doing the work as effectively as the test equipment says it should be. This happens with older vehicles, even though the catalytic converter is still working in some capacity.