My vehicle Failed Emission Inspection: Causes and What To Do

Failing emission tests can be quite perturbing, unnerving, and frustrating. Did you have a failed emission inspection and wonder ‘why the emission failure?’

A failed emission may look like the inspectors are trying tricks on you, especially when your baby ride is running nicely and smoothly. But how do you know what’s wrong with your vehicle?

This article has provided sufficient information on the reasons for a failed emission test and how to fix it. Grab a seat; let’s reveal them.

Why A Vehicle Fails An Emission Inspection

vehicle emission test

Several factors could cause emission failures. While some are easy to fix, others are complex jobs that require expensive fixes.

Read Also: Best Automotive Smoke Machine

Old Motor Oil: If you are fond of not changing your engine oil at regular intervals, it’ll not only make you fail emission; it will form engine sludge which will damage the engine over time. Smog technicians measure hydrocarbons during emission inspections, and dirty and delayed oil change accumulates hydrocarbon.

Bad Catalytic Converter:  A vehicle with a bad catalytic converter may experience overheating and emit a sulfuric smell. On its top, you may also perceive an odd smell like that of a rotten egg.

Why would a failed catalytic converter cause you to fail the emission test? It’s simple! Because it can convert carbon dioxide.

Bad Fuel Injectors: A failed or clogged fuel injector will disrupt the fuel passage in the injector lines. It can cause the air/fuel mixture ratio to be too lean or too rich.

If there is too much unborn fuel left during the normal combustion process, it’ll result in carbon dioxide in the exhaust system, which will turn to emission and leave the exhaust tailpipe.

Leaking or loose gas cap: This is common, right? It occurs more often than you can imagine. As common as it is, it can cause a failed emission inspection on Cairfax. Hence, it is important to ensure the gas cap is tightened properly after refilling fuel at a gas station.

Some vehicles will give an audible click when the cap is locked.

Faulty air intake levels: If your car failed an emission test and revealed an inappropriate level of hydrocarbons and carbon monoxide, you may have a clogged air intake system.

This is mainly a clogged or dirty air filter, which will result in a lean air/fuel mixture ratio. This literally means unburnt fuel escaping into the exhaust system and causing the exhaust tailpipe to emit greenhouse gases.

Lousy Oxygen Sensor: The 02 sensor works in harmony with your car onboard computer by regulating the oxygen levels in the exhaust system. In case this car sensor fails, it’ll send false readings to the car computer, causing an inappropriate air/fuel mixture ratio into the cylinders. This will trigger limp mode and lead to loss of engine power.

Now, how do a failed emission test MOT and lousy 02 sensors relate? When this sensor fails, the PCM will not know if the engine is operating under normal tolerances. And it will cause the vehicle to fail an emission test.

Check engine light: A check engine light on the instrument cluster, whether due to the outlined causes or other engine issues, will automatically cause your little boy to fail an emission test.

While it is quite easy to clear check engine light from illuminating on your dashboard, it can not be an escape route to pass emission tests. Instead, it’ll cause failed emission inspection not ready.

This means it will put the vehicle in a not-ready state until system sensors start sending and receiving appropriate data from normal driving.

Once the appropriate data is recorded, the system problem that triggered the check light initially will be captured, and the light will most likely reappear.

vehicle emission test procedure

How Do I Fix My Emission Failures

If your car has failed an emission test or you are concerned about fixing emission failure, stay tuned. In the next few paragraphs, we will explain how to fix emission test failures.

Check wheel tires: Some of the things to check are safety tests. Check things like tires to make sure they are not punctured, and ensure they have good tread life.

Fix the headlamps and put them in good working condition. Anybody can check the light – you don’t need a mechanic to check it.

Change engine oil: Since you can’t tell what’s coming out of the exhaust tailpipe, you can take a few preventive measures to make sure it passes the emission test.

In older cars manufactured before 1996, they measure the air that comes out of the exhaust tailpipe with a dynamometer. So, you need to put everything in good working condition to ensure the air coming out of the tailpipe is very low and not polluting much.

That said, make sure the oil is clean. Change the engine oil and oil filter before the inspection because if the oil gets dirty, the PCV valve will suck dirty vapors, burn them and pollute more.

Clean/replace air filter: Keep the air filter clean because a dirty air filter will block air passage, and the car will run lean and pollute too much.

Check the Spark Plugs: Check the spark plugs and ensure they are in good shape. Spark plugs have a gap between the electrodes and the body. If the gap is too wide, they won’t ignite as normal.

Even if the car is running okay, if the gap is too much, it’ll pollute more and fail the test.

Check the cooling system: A low coolant will make the engine run higher and, of course, result in overheating. So make sure there’s enough coolant and the engine is not overheating.

Put the cooling fan in a nice and smooth running state. Start the vehicle and turn on the AC to see if the fan is running smoothly. Because the test takes place on a dynamometer, the car isn’t moving; if the cooling fans are not running fine, the engine will run harder, and your car will fail the test.

A dynamometer is a measuring device used for determining torque or power.

For Instance, it is used to calculate the power generated by a motor, engine, or other rotating prime movers by simultaneously measuring force (torque) and rotational speed (rpm).

Older vehicles that were produced before 1996 are not equipped with an OBD11 plug-in socket. For this reason, CERTs do not test the engine with any computer other than a dynamometer

Fix engine check light: If you have a modern car manufactured from 1996 and upwards, the test will not take place on a dynamometer. They will plug a computer into the car to see if there’s a problem.

So, if the car’s check engine light is illuminating in the dashboard, get a certified mechanic to diagnose and fix the fault before you go for the test. If there’s a trouble code stored in the computer, the scan tool will detect it.

There are over 2,000 separate fault codes that can exist in a car. This makes it complicated to figure out the cause of the illuminating light without a scan tool.

Use fuel injector cleaner: If you have tried everything mentioned above and the car still fails an emission test, you can try a fuel injection cleaner and pour it into the gas tank.

I recommend using a shop-line solvent. Pour a gallon of the solvent into a half-gas tank and drive the car on the highway at maximum speed. Drive for about 30-45 minutes.

I have done this on several cars and got them to pass emission tests. Of course, this is not magic; if your car has faults, you need to fix it. But if the car runs okay, this will help it pass the test because the solvent burns clean and helps push out cleaner smoke. If you have older cars with catalytic converter issues, this will help them pass the test.

failed emission test evap


Q: What happens if I fail the emission test twice?

If your baby ride fails an emission test, it means its emission state is getting beyond the permissible emission allowed in the manufacturing year.

When a vehicle fails the emission test, it is expected to be repaired and returned for another test within 60 days of the initial test.

Now, what happens if you fail an emission inspection 3 times or twice? You may qualify for a cost waiver as long as you meet the expenditure criteria and the repair is done by a certified emission technicians CERT.

Q: Should I buy a car that fails the emission test?

In most parts of the United States, it’s illegal to sell a car that doesn’t pass the current emission test. If a car you intend to buy has multiple failed emission test records or doesn’t have current records for up to a year, insist the car go for an emission test or get another car.

Q: How much does it cost to fix emissions?

How much to fix emission problems depends on the cause of the failure. However, if a faulty oxygen sensor or EVAP system is diagnosed as the culprit, the repair and diagnosis cost should be $100 to $200 per vehicle.

Q: How many miles do you have to drive to pass smog?

Remember that internal combustion engines run on fuel. Drive your car 10-15 miles before getting to the smog station. This will allow the engine to warm up and ensure the catalytic converter, motor oil, and coolant run at the normal operating temperature. It’ll also aid your car to pass the emission test as well.

One of the emission inspection standards requires a smog technician to check and ensure the vehicle runs at normal operating temperature before running the inspection. However, this is one of the prerequisites to pass emission inspection that you can do with ease.

Q: How do I get a waiver for emission?

There are three types of emission waivers. Here’s how you can get any of them;

Cost waiver: You are expected to meet maximum expenditure prerequisites on emission repairs. The repairs must be done by certified emission repair technicians CERT.

Economic hardship: To get a hardship emission waiver, you are expected to submit documentation to DMV stating your annual income is below the federal and state poverty state level.

You will also attest that you have no other income or assets to pay emission repair costs.

Functional diagnosis: There must be documented reasons for the failed emission failure, and the documentation must come from the manufacturer and not from a dealer.

The letterhead must state the reasons why the car cannot pass or is failing the emission test.

Q: What happens if I buy a car that doesn’t pass smog?

If you purchase a used car and it doesn’t have a current smog test record, the dealer is required by law to fix the car to pass smog.

Q: Can I still drive with emission problems?

As long as it is only emission problems you are experiencing at the moment, you can still drive without worrying about safety. However, you should diagnose and fix the faults before it escalates to an expensive repair.

Final word

Emission tests help ensure that only worthy vehicles are driven on the road. This is to make sure that vehicles emit less emission pollution into the atmosphere to save the environment and its habitants.

Having known the reasons for emission inspections, if you fail an emission test, get it fixed for the benefit of the road users.

Now, you know the reasons for a failed emission inspection and how to fix it, contact your local mechanic to fix it or drive down to CERT for a professional fix.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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