No Check Engine Light but Failed Emission Test – What's Wrong?

An emission test is necessary if you live in a place that requires smog or emission testing. Emission tests are compulsory because your exhaust system might be in bad condition that could release toxic fumes that are harmful to the air quality of the atmosphere.

Most of the time, you might be worried about an emissions test failure when your vehicle seems to be running fine. What could be wrong if there's no check engine light and you failed an emission or smog test?

In short, there could be multiple reasons you failed an emission test with no check engine light on. If you have an older model vehicle with an OBD system, loose connection, or poorly wired electromagnetic compatibility (EMC), it might be a significant reason for a failed emission test.

In this article, I'll cover the following:

• Can your vehicle throw codes without a check engine light?
• Common reasons for a failed emission test
• Why do older cars with an OBD system fail emission or smog test

Can your vehicle throw codes without a check engine light?

As baffling as it might sound, your vehicle can throw trouble codes without signaling the check engine light. 

If your engine light doesn't show trouble codes, you may assume your vehicle emission system is in good condition. 

The check engine light is not directly connected to the mechanism displaying trouble codes.

In some cases, an open connection or wiring crisis might cause the check engine light not to come on.

Do you need the engine light code to be on before you can get trouble codes?

This is another common question about engine light codes. Most people fail their emission tests because their check engine light fails to detect trouble codes from their exhaust system before going for a test.

Meanwhile, most people believe your check engine needs to be on before you can have trouble codes, which is only partially true. 

Every modern vehicle has a computer system that consistently collects and analyzes data. Also, the system monitors the car's operation and provides information that helps the vehicle work in top condition.

Most components in the vehicle are tracked. Oil pressure, gas mileage, fuel filter, and electrical systems are analyzed and stored.

So, with the assistance of an automotive technician and a proper diagnostic scanner, trouble codes and data in your vehicle can be detected without your check engine light on.

Common reasons for a failed emission test

Do you have a vehicle operating smoothly, but you somehow managed to fail your emission/smog test?

These are the common reasons for a failed emission test.

• Dirty air filter

Harboring a clogged or dirty air filter in your vehicle could increase hydrocarbon and make you fail the emissions test.

Modern vehicle manufacturers recommend you change your air filter every 12 months or after every 12,000 miles.

If you've exceeded the recommended 12,000 miles, it may be time to change your air filter. 

• Rich air-fuel mixture

A rich air-fuel mixture is a common reason for a failed emissions test. A defective oxygen sensor or injector causes this failure. 

So, most times, you are likely to fail an emission testing procedure if you don't change and replace them entirely. 

• Exhaust system problem

Generally, if there's an issue with your vehicle's exhaust system, you will fail the emissions test. Problems like oxygen sensors, air flow intake, and clogs could cause overheating if your exhaust manifold leaks.

Can older vehicles with an OBD system fail emission or smog tests?

Tailpipe is a comfortable way to diagnose a 1996 or older vehicle model. Older vehicles with the OBD system are easier to diagnose with a check engine light. So, older cars are likely to fail an emission test. 

Vehicles equipped with the OBD-2 system are built with sensors in the internal component indicating a problem. 

So, with the OBD-2 system, your vehicle can connect to an OBD-2 scan code and detect a harmful exhaust condition before an emission test. 

With the OBD-1 system, you're at a disadvantage before going for an emission test. That is why you're less likely to get more information from the computer than the 1997/98 and newer car models.

Unfortunately, unlike the OBD-2 system in modern vehicles, the check engine lights in older cars with an OBD-1 system are diagnosed differently.

However, I will share some helpful tips that will make your vehicle perform smoothly during the testing operation.

How to pass an emission test with an OBD-1 system

1) Properly inflate your tires

Properly inflated tires will reduce pressure and additional drag on your vehicle, ultimately reducing emissions. 

Suppose you often inflate the tires of your vehicle pressure poorly. In that case, you're unknowingly reducing your gas mileage and causing carbon dioxide emissions, which are poisonous and harmful to the environment's air quality.

According to research, having an adequately inflated tire will reduce your engine's carbon by 327 g per year.  

If you're still rocking your older model vehicle with the OBD-1 system, incorrectly filling your tire pressure could be one of the reasons why you failed the emission test. 

You can often spot the recommended tire pressure on your vehicle's side door jam. 

So, next time you take your vehicle for an emission test, ensure you adequately fill your tire pressure.

2) Ensure The Gas Cap Is Well Tightened

The loose gas cap of your vehicle could be one of the reasons for a failed emission test in an old car with the OBD-1 system.

However, a broken, open, or poorly sealed gas cap produces toxic fumes that could cause engine crises. Also, if your gas cap is faulty and isn't absorbing vapor, it will result in a dry Evaporative Emission control (EVAP) system. 

An EVAP is the vehicle's system that prevents harmful fumes from leaking into the atmosphere.

The good news is gas caps are affordable and can be easily replaced.

The average price of a gas cap sits between $92 - $97. The repair and labor costs are evaluated between $18 - $22

3) Engine Warm-up 

Before you go for an emissions test, you'd need to be prepared. Some minor adjustments you overlook might be one of the causes of an emission test failure.

Engine warm-up is very critical before test procedures for an emission test. That is why after igniting your engine from a chill temperature, let it warm until it reaches peak temperature. 

The fuel mixture stays rich when the engine is chilled. As the coolant and the oil reach their peak temperature, the air-fuel combination optimizes for better performance and reduces the unburnt fuel emitting in the form of hydrocarbon.

4) Engine Oil Change

Changing your engine oil is one of the most important ways to pass an emission test with an OBD-1 system.

Passing an emission test requires all the boxes related to the exhaust system check. Frequent engine oil changes will reduce the emission of poisonous fumes from your engine. 

With the support of the catalytic converter, toxic fumes emitting from the exhaust system are converted into less harmful smoke.

In a nutshell, unchanged dirty engine oil will enhance the emission of hydrocarbon fumes and poor performance during testing operations. 

So, an oil change is a significant boost before an emission test. 

5) Engine Tune-Up 

Vehicle maintenance, known as engine tune-up, is another important way to pass an emission test on your older model OBD-1 system.

Practicing scheduled maintenance will help you squeeze every gasoline drop in your vehicle. 

Also, vehicle maintenance will help you examine impaired spark plugs, air, and fuel filters. And failure to replace the defective items might cause your vehicle to burn more fuel and work harder, which increases emissions of harmful fumes.

Moreover, vehicle maintenance will curb emissions and increase your vehicle's performance and condition.

6) Vacuum leaks 

Vacuum leakage in engines can be complicated because of its array of wide-body rubber elements. 

On most vehicles, the air intake is metered before it's released into the engine, and the oxygen sensor measures the flue gas. 

Over time, the rubber components will get worse and deteriorate, which will cause an additional air intake into the engine. 

The oxygen sensor will detect this malfunction and send a signal to the engine's computer to add more fuel mixture, which causes more emissions.

If your rubber component continues to deteriorate, it's better to take it for repair if you want to pass your emission test. 

7) Replace Oxygen Sensors 

When preparing for an emission or smog test, and you have a vehicle with over 100,000 miles, there's a good chance the oxygen in the car's sensor is defective and needs to be changed.

As the oxygen sensor wears out, you'll notice symptoms of decreased fuel consumption and sluggish performance.

Replacing the impaired oxygen sensor with a new one will restore your vehicle's peak performance and reduce unwanted emissions.

Conclusion

An emissions test is critical and requires careful examination. So, when you fail an emissions test with no light engine on, it can merely be a loose connection in your engine light, which fails to detect emissions difficulties before going for a testing procedure. 

Also, if you have a 1995 or older vehicle with the OBD-1 system, you don't get enough information from the computer to detect engine light codes before an emissions test.