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7 Real Reasons Your Car Won’t Pass Inspection and How to Fix Them

Marc Skirvin
Marc Skirvin

What's in this Article

About 7% of light-duty vehicles fail emissions tests each year. That amounts to 133,104 automobiles out of 1.9 million.

The odds are certainly in your favor, but given the recent busts of popular car brands in cheating tests, it’s best to be prepared for your emissions test. Especially if you have a gas guzzler or older vehicle.

So what should you look for and why should you bother? We’ll cover it all here so you can adequately prepare for your own inspection and avoid having your car turn into an unusable eyesore in the front yard.

The Importance of Minimizing Pollution

Did you know you’re contributing 4.6 metric tons of carbon dioxide each year just by driving your vehicle? To give you some perspective, that amounts to 10,141.3 pounds.

And the average family has two cars, meaning they’re contributing over 20,000 pounds of carbon dioxide in their travels alone.

No one expects you to hike 20 miles to get to work, but it’s impossible to deny the impact automobiles have on the planet. So when the annual inspection time rolls around, just remind yourself it’s for a good cause.

What to Expect

On the day of your inspection, your vehicle will undergo several tests. Always check your state laws for specifics, as some locations have rigorous standards.

Visual Inspection

Before any actual testing, expect a visual inspection. The examiner will check for leaks and safety hazards. They will also ensure your pollution control system is working.

The Real Deal

Next, your car will undergo an emissions test. How it is tested varies depending on the car year and model. Newer vehicles usually have an onboard computer that stores relevant emissions information, so all it takes is a quick connection to retrieve the data.

Other automobiles require a dynamometer or idle test. A dynamometer test requires you to drive on a large treadmill-like machine at a particular speed. The examiner uses a device to measure the pollutants exiting your tailpipe.

The idle test works in the same way, except your car remains stationary and idling. You rev the car while the examiner measures pollutants.

Gas Caps

Finally, you’ll make it to the gas cap test, which ensures your cap doesn’t let harmful vapors into the environment. The examiner will use a tool to measure its performance, which determines if your car passes.


If your car fails any test, you won’t be able to renew your license and registration. Since most tests parallel renewal dates, that means no driving.

7 Reasons for Emissions Test Failures

From clogged air filters to faulty spark plugs, reasons abound as to why your vehicle might fail its emissions test. On a good note, you’ll be surprised to find quite a few of them are easy (and inexpensive) fixes.

1. Clogged Air Filters

The air filter is an overlooked component in a car. However, these small parts play a vital role in how smoothly your engine runs (or doesn’t run).

An air filter catches all the dirt and grime that travels into the hood of your car and towards the engine while you drive. If the filter becomes too dirty, the engine is deprived of air. As you can imagine, this impacts engine performance.

Air filters are so effective at their job some scientists suggest using them to study pollution levels in inner cities.

If the air filter is dirty, it is unable to stop hydrocarbons and heavy metals from filtering through your automobile. Consequently, you could fail the test by having high levels of hydrocarbons.

This is a quick, inexpensive fix. Pick up the air filter for your make and model at a local store and replace it. While air filters can be cleaned (usually with a vacuum or damp cloth), it’s safer to simply replace it with a new one before an inspection.

After all, it doesn’t cost much and only takes about five minutes.

Filters generally last between 15,000 and 30,000 miles, but you can always visually inspect it to see how dirty it is prior to your appointment.

2. Defective Catalytic Converters

Properly functioning catalytic converters are essential for passing your exam. These devices transform the pollutants coming out of your engine into steam.

catalytic converter is comprised of a box and two pipes. One pipe allows hot, polluted fumes from the engine to pass into the box, where it undergoes a chemical reaction that breaks down the pollutants into safer gases. These gases are released through the second pipe and out into the environment.

Without a catalytic converter, the pollutants from your engine are escaping directly into the environment. There is no instrument making them safer.

Since the 1970s, catalytic converters were used in manufacturing. You probably won’t pass a test without one.

Faulty catalytic converters are difficult to replace for novices. It requires taking off the old converter (which is sometimes welded to the car) and putting in the new one.

Consequently, we recommend seeking the help of a professional.

Replacements cost anywhere from a few hundred dollars to $2,000.

3. Dirty Engine Oil

Engine oil helps avoid harmful friction between parts. It’s necessary for an automobile to run efficiently.

When engine oil becomes dirty, it holds onto contaminants and pollutants. That means it adds to the levels spouting from your tailpipe.

Oil changes are simple processes that only cost about $20 if you do it yourself. DIYers remove the drain plug to release old oil and add fresh oil to the engine. It only takes about 20 minutes, but it can also be done at a low price at a local shop.

4. Improper Air-to-Fuel Ratio

When optimized, the air-to-fuel ratio provides the perfect mixture of fuel and gas for safe, low-emission combustion. The proper ratio is 14.7 parts of air to one part of fuel.

If the ratio is incorrect, your fuel won’t combust properly. That means you’ll have extra pollutants and burn through fuel.

There are several reasons your AFR might be off, so it’s best to have your vehicle checked by a professional here. With new sensors on the market, however, carburetor tuning is possible for the brave of heart.

5. Broken Oxygen Sensors

The oxygen sensor is a necessary component for appropriate air-to-fuel ratio and for the emission control system. It’s in charge of keeping an eye on that ratio we discussed a moment ago.

If the O2 sensor is broken, your vehicle won’t make the necessary adjustments to the air-to-fuel ratio. That means it spews out more pollutants than it should.

Fixing an O2 sensor by yourself involves discovering which one isn’t working (many cars have multiple sensors), unscrewing it, replacing it with a new one and clearing the codes in your vehicle’s computer.

In some instances, the new sensor doesn’t have an adaptor. If this is the case, the process requires some rewiring. In these scenarios, it’s best to seek the aid of a professional because fusing the wires can cause further damage.

6. Leaking Gas Caps

leaking gas cap means immediate failure in an emissions test, so it’s best to ensure yours is working properly. A gas cap contains harmful fumes from your fuel that would otherwise leak into the atmosphere.

Approximately 17% of US vehicles on the highways have missing or misused caps, which add 476,000 tons of vaporized hydrocarbons and 147 million gallons of vaporized gas to the air each year.

Without this barrier, the fumes simply leak from your vehicle.

There are $5 replacement caps that you can pop on to the area where you insert gas. If you’re worried about your present cap, local stores also offer gas-cap testers for affordable prices.

7. Faulty Spark Plugs

As the name implies, a spark plug sparks to ignite the fuel and create combustion. Without this part, your car won’t start.

When a spark plug doesn’t work correctly, it causes incomplete or uneven combustion, which then affects the air-to-fuel ratio. Consequently, your fuel may be too lean or too rich, which contributes more pollutants to the environment.

Spark plug replacement is a difficult and lengthy process. If you’re comfortable removing parts of the manifold, it might be a do-it-yourself project you’d be up to tackling. For everyone else, however, it’s best to take your vehicle to your local mechanic.

Other Tips for the Big Day

Other tips for the big day include avoiding wet weather, which causes more stress on your engine and may create higher pollutant levels, and taking a leisure drive before heading to the exam site. The latter tip is because it gives your vehicle time to get to the ideal operating temperature.

Finally, fill your tires. It helps your engine maintain a consistent load if you have to drive on the dynamometer.


Inspecting these seven characteristic prior to your emissions test is the best way to avoid a failing score.

But let’s face it. If you have an old junker that coughs out pollutants like it has a bad case of bronchitis, it might be time for an upgrade. However, that doesn’t mean you can’t get some funds towards the new vehicle.

Our company can provide a free cash offer for your automobile! Visit our site to find out how much money you could get. You’re one step closer to that shiny new vehicle and no imminent worries about pollution.

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About the Author


Marc is the Co-Founder of Cash Auto Salvage and Director of daily operations. He retired from a leading Internet Marketing company in 2013 and has been involved in the automotive industry ever since.

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