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10 Reasons Why Your Check Engine Light Is On

by Bill Shaffir 3 years ago in list

Your check engine light can indicate a range of issues with your vehicle, from minor to major.


Whenever something goes wrong with a vehicle, the first reaction many car owners have is to start freaking out. Likewise, once that pesky check engine light lights up on the dashboard, people start presuming the absolute worst.

For some people, the check engine light automatically has them thinking that their vehicle will suddenly fail on them in the midst of driving. Others might be more worried about the potential hefty costs they believe they will have to pay to have whatever the issue is resolved.

Before getting anxious, know that a check engine light can indicate over a dozen different issues with your vehicle, from a loose gas cap to low oil pressure to a damaged oxygen sensor. Take a deep breath, pay attention to other vehicle symptoms, and slowly troubleshoot the issue.

Worn out Spark Plugs or Ignition Coil

Apart from an activated check engine light, if you're experiencing lack of acceleration, a rough idle, difficulty starting your vehicle, an engine surge or misfire, or high fuel consumption, it's possible your spark plugs are worn out and need to be replaced soon.

A worn out ignition coil may present the same symptoms as a worn out spark plug. However, because it can potentially cause your car to turn off without warning, it's crucial to get this part replaced right away. Replacement coils cost around $50. Spark plugs are around $10 to $20.

Remember, if your vehicle runs on diesel, rest assured that a worn out spark plug or ignition coil is not a possible issue, as your vehicle will not have these parts.

Damaged Vacuum Hose

As vacuum hoses get older, they tend to dry out and get cracked, especially in the cold or high heat. A damaged vacuum hose can cause a plethora of engine problems, including engine backfiring and the engine either not starting or losing power.

Fortunately, your local auto parts store will have vacuum hoses for just a few bucks. While it's a quick and cheap fix, replacing a vacuum hose is crucial in that if you don't do such in a timely manner, you could receive poor fuel efficiency or very serious, expensive engine damage.

A Missing, Damaged, or Loose Gas Cap


Did you know that the most common issue check engine lights come on for is due to a loose gas cap? CarMD found that a whopping 46 percent of newer vehicles with their check engine light on have nothing more than a gas cap that needs to be properly tightened. Easy peasy.

However, according to AutoZone, a loose gas cap can account for up to 30 gallons of gasoline leaking over the course of a year.

Even if your gas cap is already on nice and snug, it's also possible that your gas cap needs to be replaced. Many auto parts stores offer replacement gas caps under $15, or for a little more if you want your cap to have a lock.

A Dirty Air Filter

In extreme cases, a very dirty air filter may also cause your check engine light to come on. Without a clean filter, your vehicle's engine will be restricted to the proper air intake. As a result, the air and fuel mixture inside the engine will be altered.

In turn of the latter, the improper fuel/air mixture in your engine can result in other problems, such as incomplete combustion, engine misfire, or faulty turbocharger performance. For that reason, regular air filter maintenance is crucial.

Failing Catalytic Converter

A failing catalytic converter is another issue that your car may have if your check engine light is going off. A working catalytic converter is important, considering that it turns carbon monoxide into carbon dioxide through the combustion process in all gas-running, modern cars.

When deciding if you have a failing catalytic converted, look out for discolored smoke or odd sounds coming from the exhaust. To prevent a clogged catalytic converter, knowing how to change your oil is important. Otherwise adding $40 or $50 to the replacement catalytic converts, which costs around $300 to $600 by itself.

Problems with an Aftermarket Alarm


If improperly installed, an aftermarket alarm can pose issues for your vehicle. Apart from a sensitive and/or malfunctioning car alarm, a poorly-installed aftermarket alarm can prevent your car from starting, drain your battery, or even turn on your check engine light.

To resolve this issue, you might need to have your aftermarket alarm reinstalled, fixed, or even completely replaced. Better yet, if one can pay a little more to have their aftermarket alarm installed the right way the first time around next time, all the better.

Damaged Oxygen Sensor

Oxygen sensors, or O2 sensors, are important in that they work by comparing the ambient (outside) oxygen to the oxygen within your exhaust system. The level of oxygen present in your exhaust system can indicate the amount of oxygen and fuel mixture infiltrating your engine.

However, a damaged oxygen sensor can cause more fuel to burn than usual. It may also damage other parts, such as the spark plugs or catalytic converter, if left untreated long enough. An oxygen sensor replacement costs around $175. Most modern cars have multiple oxygen sensors.

Faulty Mass Air Flow Sensor (MAS)

With a poor mass air flow sensor (MAS), your engine will not be able to understand the correct amount of fuel it needs. In turn, this can negatively impact your mileage, cause rough idling, make your vehicle lose power or have trouble starting, or cause altitude adjustment problems.

To replace a faulty mass air flow sensor, the cost would be approximately $120 to $150, depending on the make and model. As the MAS is replaced, a mechanic should also inspect the vacuum hoses and snorkel attached to your sensor for potential leaks.

Dead Battery


If your check engine light comes on, and your car won't start, charge your phone, and your lights won't turn on, a dead battery is likely the culprit. Fortunately, charging or even replacing a dead battery is typically both easy and affordable.

While dealing with a dead battery is not a difficult task, many modern cars have a strategically-placed battery in a difficult, hard-to-reach location so that the average mechanic cannot figure out how to disconnect your car battery. As a result, this issue may only be spotted and fixed by the vehicle's manufacturer for a heftier cost.

Faulty Evaporative Emissions (EVAP) Purge Solenoid

According to CarMD, one of the most common check engine light repairs in 2015 was to replace a faulty EVAP purge solenoid. This part is crucial to ensure your vehicle isn't expelling too many pollutants into the environment.

Symptoms of an EVAP purge solenoid that needs replacing, besides activation of the check engine light, may include difficulty starting or rough idling. However, a vacuum leak can also cause these issues, thus, it would be important to consider both as potential diagnoses.

It may be overwhelming to think about all of the potential issues your check engine light may represent, but the only way to know for sure what the trouble is, is to seek out someone who can help troubleshoot your vehicle if you are unable to do so yourself.

Taking your car to a professional or purchasing an OBD II scanner that can read your vehicle's trouble code(s) (and pull up vital vehicle information such as mileage) are important steps to fixing these problems. Receiving the diagnostics system report, you can know exactly what needs to be done going forward. Who knows? If you keep up with some of the car maintenance tips every new driver should know, you may never run into this problem!

Bill Shaffir
Bill Shaffir
Read next: 'Stop! We're Going to Crash'
Bill Shaffir

Lover of gangster films and worried about the future of artificial intelligence.

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