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Most Common Causes of a Check Engine Light and What You Can Do

Check engine lights have a way of sticking in every owner’s mind. Sure, there are some owners that sleep well at night even when their check engine light is on, but there are a lot of people who can’t sleep when their auto has a problem.

The good news is that a high-end diagnostic tool can read the error codes your check engine light is trying to tell you.

Our Fixd review goes into great detail on how to use a diagnostic tool to remedy your check engine light.

But if you don’t have Fixd yet and want to know the most common reasons for a check engine light coming on, that’s what we’re going to discuss.

5 Most Common Causes for a Check Engine Light

1. Oxygen (02) Sensor

An O2 sensor is vital to your vehicle’s operation, and this sensor is the main reason that check engine lights come on. The bad news is that these sensors are rather expensive to repair, with the average cost being $246 with parts and labor.

The O2 sensor must be replaced if you don’t want the following to happen:

  • Burn excess fuel
  • Lose up to 40% fuel efficiency
  • Damage your catalytic converter or spark plugs

If you have a single exhaust, you’ll likely have 2 oxygen sensors. If your vehicle has two exhausts, there’s a good chance that you have 4 oxygen sensors. You may need to replace more than one, depending on the diagnostic code and what the mechanic tells you to do.

You never want to ignore a check engine light because a simple fix can become a very costly problem if it’s allowed to progress.

The oxygen sensor is vital to the catalytic converter. If you allow the problem to continue, a $200 repair can swell to a $2,000 repair.

2.  Gas Cap Issues

Gas caps are also monitored by your onboard diagnostic system, and there are several times when a gas cap can lead to the check engine light coming on:

  • Loose cap
  • Missing cap
  • Worn cap

Your gas cap helps your automobile maintain its pressure in the fuel tank. A missing or faulty gas cap can’t do its job properly. If you choose not to replace your cap, your vehicle will suffer from:

  • Excess fuel loss

The seals around an old gas cap have a tendency to dry out and become worn. There’s also a chance that you’ve left the gas cap somewhere and it’s no longer on your vehicle. This is an easy fix.

Replace the cap.

You’ll find most auto stores sell gas caps. You can also go to a mechanic to have them replace your gas cap, but this is a costly fix that would take you seconds to do yourself.

3. Catalytic Convertor

Your catalytic convertor is what helps you protect the environment from your vehicle’s exhaust system. See, this convertor will convert all of the carbon monoxide that your vehicle produces into carbon dioxide.

The bad news is that the convertor can become damaged due to:

  • Bad oxygen sensors
  • Mass airflow sensors
  • Spark plug / wire issues

As mentioned earlier, this is a serious repair and a very costly one. The prices really do vary from $300 – $3,000, depending on the type of vehicle you own. The average cost to repair a catalytic convertor is $1,029.

If you fail to replace your catalytic convertor, you’ll notice that your vehicle will:

  • Not pass emissions tests
  • Suffer from performance loss
  • Suffer from fuel economy loss
  • Run at a higher temperature

A complete failure will eventually force you to completely replace your catalytic convertor. Your auto’s gas mileage will suffer dramatically.

The bad news is that this is a difficult repair, so you’ll be at the mercy of a mechanic if your catalytic convertor does fail.

4. Mass Airflow Sensor

Mass airflow sensors, or MAF sensors, are designed to allow air to enter into the engine. The goal of the mass airflow sensor will be used by the engine to determine how much fuel your engine needs to run efficiently.

If the sensor is damaged or faulty, it can cause:

  • Damage to your O2 sensors, spark plugs or catalytic convertor
  • Cause your vehicle to suffer from fuel economy and performance loss

You can drive around with a broken or faulty MAF for weeks without causing too much damage, but it may eventually lead to your car stalling. There’s also a high likelihood that the vehicle’s gas mileage will decrease over time.

Replacing your MAF will cost $200 – $300 at most shops, depending on the type of car that you own.

The good news is that this is a quick fix that most people can do on their own. The only downside is that if the MAF isn’t the issue, you’ll need to diagnose the issue yourself.

5. Spark Plugs

Your spark plugs are so important to the operation of your vehicle that it won’t perform well or will misfire they fail. A vehicle’s spark plugs ignite the air/fuel mixture that is also helped by your MAF. When the combustion occurs, a spark from your vehicle’s ignition coil will be sent to your spark plugs.

If your spark plugs or wires are bad, your vehicle may suffer from:

  • Poor performance
  • Reduced power
  • Poor fuel economy
  • Ignition coil damage
  • Damage to your O2 sensors

Your vehicle will likely misfire due to the bad spark plugs.

The great thing about spark plugs is that it’s rather easy to change your spark plugs yourself. A few points to keep in mind are:

  • Spark plugs need to be changed every 25,000 – 30,000 miles for vehicles made in 1996 or earlier
  • Spark plugs can last up to 100,000 miles on newer vehicles

Of course, spark plugs can fail and need to be replaced at any time. You should replace all of your spark plugs when one goes back – you’re working on them anyway. You’ll be able to access your spark plugs right from under the hood, and swapping out your spark plugs should take just an afternoon.

You don’t just want to start replacing random parts and components. Use a code reader before swapping out parts to make sure that you’re replacing something that actually needs to be replaced.

August 18, 2017


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Picture this: You’re driving down the highway, when all of a sudden your check engine light comes on. Your car had been driving a little funny, but nothing too out of the ordinary. In a panic, you pull over off the road and call a tow truck.

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