Top 5 Reasons Your Truck Failed the California Smog Test

You need your truck to make a living, but you need clean air to live. 

Emissions tests were developed with this in mind and are common in many states, part of an effort to reduce the harmful emissions associated with the transportation industry, which account for 28% of total US greenhouse gas emissions

California’s emission test is called a Smog Inspection and is required on all vehicles, except cars built before 1975, and diesel trucks with model year 1997 or older, or those with a gross weight of over 14,000 pounds. 

If your truck doesn’t fall within these guidelines, you’ll need to pass an annual Smog Inspection to renew your registration. 

While these tests have been designed with the greater good in mind, passing a semi-truck smog inspection is a bit of a hassle, especially if you’ve failed one but don’t know what the problem is. To learn more about how to pass a semi-truck smog check, keep reading.

What Is Smog?

Smog is a combination of smoke and fog. It’s formed as a result of sunlight coming into contact with nitrogen oxide and other organic substances that are found in fossil fuel emissions from vehicles, power plants, and factories.

Smog is harmful to the environment, plants, animals, and humans. It can also cause corrosive damage to buildings and vehicles.

Why Trucks Fail Smog Checks 

If your truck fails a vehicle emission test, you won’t be able to complete your auto registration. As you probably already know, outdated registration tags can lead to police tickets, insurance problems, issues with your employer or contractor, and other consequences.

Numerous factors could cause a failed truck emissions test. Here are some of the most common reasons: 

1. A Leaking Gas Cap

Thankfully, a leaking gas cap is an easy and inexpensive fix. 

If the truck’s gas tank cap is cracked, incorrectly sealed, or is suffering from excessive wear and tear, it can increase emissions. If it increases your emissions output too much, a leaky gas cap can cause you to fail your Smog Inspection. 

2. Faulty Ignition System

A faulty ignition system is usually caused by old, worn, outdated, or defective spark plugs. It could also be due to faulty wiring. 

Anytime a vehicle’s ignition system isn’t operating properly, it releases excess hydrocarbons, which can cause your vehicle to fail an emissions test. 

3. A Dirty Air Filter

The air filter is something that people tend to forget about, especially once the weather cools down and air conditioners stop blasting. 

But a dirty air filter can cause a lot of air quality problems, both inside and outside the cab.

A clogged, dirty air filter can’t filter the air coming out of the engine and can cause a truck to fail an emissions test. 

Also, if you’ve noticed air quality inside your vehicle seems poor- you’re coughing or sneezing a lot, have itchy eyes, or a sore throat, it could be that your air filter just needs a change. 

Fortunately, swapping out a dirty air filter is a quick, easy, and pretty inexpensive fix. 

4. Faulty Evaporative Emission Control System

The Evaporative Emission Control System (or EVAP system) is designed to prevent vehicles from releasing fumes from raw gasoline into the environment. 

While the exact design varies based on the manufacturer, they all contain a vent line from the gas tank to a charcoal vapor canister, which will then vent into the atmosphere. 

The problem with EVAP systems is that they consist of multiple parts, leaving multiple opportunities for breakage. A semi-truck inspection test can determine if the issue lies with the vacuum hoses, purge valves, or another EVAP system component.

5. Malfunctioning Catalytic Converter

The purpose of a catalytic converter is to convert the hydrocarbons, nitrogen oxide, and carbon monoxide that is released by the engine into water and carbon dioxide.

A faulty or leaky catalytic converter will cause unfiltered toxic emissions to escape, causing you to fail an emissions test. 

A catalytic converter is a relatively small piece of equipment, but the intricate manufacturing process and materials used in its construction (many require the use of precious metals, like platinum), mean a malfunctioning catalytic converter isn’t cheap to replace. 

How to Pass a Vehicle Emissions Test

Don’t let this top 5 list get you down; there are several ways to increase your chances of passing a smog test. Here are a few suggestions:

Get an Oil Change

Dirty oil is loaded with hydrocarbons that can trigger a failed smog check. Before you go for an emissions test, get an oil change. 

An oil change is also a great opportunity to ask your Kelly’s Truck technician to give your truck a quick inspection to make sure there are no major broken, cracked, or disconnected hoses that may cause a test failure. 

Fix that Check Engine Light

If your check engine light is on, your vehicle stands a good chance of failing an emissions test. Thankfully, correcting a check engine light usually pretty straightforward. 

Check out our article on “Why the Check Engine Light is On”, and bring it to Kelly’s for a checkup. It will be time well spent, especially if it catches a problem that would have caused you to fail the emissions test. 

Get a Tune-Up

A tune-up ensures that your engine is in good working order. However, you shouldn’t get it done right before the smog test.

During a tune-up, most mechanics will disconnect the battery, which resets your vehicle’s computer. So if you need a tune-up, it’s best to do it a few weeks before your smog check and drive the vehicle for at least 100 to 200 miles before the test.

Make Sure the Battery Hasn’t Been Disconnected

In addition to tune-ups, you also want to make sure that your truck battery hasn’t recently been disconnected, as this will cause the on-board computer to lose power. If you had a jump-off or replaced the battery, wait at least a week before going to get your emissions test.

When a vehicle computer loses power, it erases the internal self-test monitor, which includes the emissions monitor – a red flag at your emissions test. But once you drive at least 100 miles, the emissions monitor rejuvenates itself. 

Why It’s Important to Pass a Truck Emissions Test the First Time

Failing a semi-truck inspection is a headache. Not only that, but it’s costly and time-consuming. Some emissions testing centers charge at least $20 per test or more.

If you have to go through testing several times before passing, you could spend roughly $100 just to get permission to reinstate your vehicle registration.

Not only that, but it takes time out of your schedule and away from your routes to get the tests done. If you don’t pass the first time, that means spending multiple hours going through emissions in hopes of passing.

It’s better to have regular maintenance on your truck than to wait until it’s time to get new registration and have to fight with emissions tests and last-minute repairs. 

Pass Your Semi Truck Smog Check With Ease

If you want to make sure that you pass your smog check for your upcoming emissions test, Kelly’s Truck repair can help. Our extensive parts department and in-house body work shop means that we’re your one-stop-shop for truck, trailer, and fleet repairs.  

If you have questions about a semi-truck smog check or other inquiries, call us at 1- 800-793-9282 or contact us online.

We look forward to getting your truck running smoothly.

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