How to pass smog tests

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5 min read

Smog tests—also called emissions tests—help keep the air we breathe so fresh and so clean, clean. In most states, they are a regular requirement for cars and were introduced in the 1970s as part of the Clean Air Act. They're important for the environment and everyone's health. It's definitely a test you want to pass.

Not all states or cities require smog tests (check if yours does here), but if so then you might have got a notice letting you know that your car is due one. Usually these need to take place every year. You might also be wondering what it’s all about, so let’s run through the process, and what you can do to improve your chances of success.

what happens in a smog test?

The smog test usually only takes an hour or two, and costs around $40. It’s all about checking that your car’s emissions system is working properly. There are a few steps to it. The technician will do a visual inspection of the car, then check the emissions system is functioning as it should. They will also conduct an emissions test to measure the levels of pollutants such as carbon monoxide, carbon dioxide and sometimes nitrogen oxides too.

what could cause my car to fail a smog test?

Even if your car seems to be running smoothly, there are some common reasons why it might fail. (But don’t worry too much, these are usually pretty cheap to fix).

bad oil

One reason that could cause a car to fail is that the engine is due an oil change (clean oil means cleaner engine and less emissions).

old spark plugs

A failed smog test could also be down to the spark plugs being worn out (also cheap to change). Old spark plugs could lead to increased emissions due to incomplete combustion.

dirty air filter

This can cause higher levels of hydrocarbons being emitted by the vehicle. It’s another easy fix and you should be changing these every year or so anyway.

faulty catalytic converter

The catalytic converter is the part of the engine that filters dangerous emissions through the exhaust. If it’s not functioning properly, it’s an instant fail. The good news is that it’s rare for the catalytic converter to need repair or replacement (but if it does it can be extremely expensive).

broken oxygen sensor

A broken oxygen sensor could result in too much fuel going into your combustion chamber, leading to an increase in carbon in the exhaust. If your “check engine” light has flashed up on the dash lately, this could be why.

tips to help you pass a smog test

If you want to avoid the hassle of a failed test then there are some simple things you can do to improve your chances.

do a pre-test tune up

Most of the reasons to fail are quick-fixes. If your car is due an oil change, or a new air filter, get that replaced before the smog test. And if that “check engine” light is on, get it looked at as your oxygen sensor may need to be replaced. Some auto stores even offer a service where they’ll give your car a quick, cheap, check to see if there are any red flags before you go in for the real deal.

go on some highway drives

For a week or so before the smog test, take your car for some highway drives. This will get the catalytic converter fired up and burn out any residues that could cause problems during a smog test. Shorter trips at low speeds won’t warm up the catalytic converter enough to have the same benefit.

use a fuel additive

Fuel additives help clear out carbon deposits, improve the airflow through your engine, and help reduce emissions. Just add some in when refueling.

prepare for test-day

On test day fit in a 20 minute drive at highway speed to get the catalytic converter warmed up. Clear out the trunk as extra weight can contribute to a failure. Some people suggest avoiding rainy days, as it could cause the car to slip on the dynamometer, which the car runs on during the emissions test (but the jury is out on this one!).

Most cars pass with flying colors, but if a smog test has revealed a ton of issues with your car—and you’re done trying to fix them—then it may be time for an upgrade. Here at Peddle we’d love to buy your car whether it passed its smog test or was a flat out failure!

Check out our instant offer tool to find out how much you could get for yours today.