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What’s the Deal with This Service Engine Soon Light? Everything You Need to Know!

Service Engine Soon Light

We’ve all had it happen. You start your car in the morning, or are driving down the road when suddenly, a light on your dashboard illuminates saying “Service Engine Soon.” Immediately your mind is filled with doubt, uncertainty, perhaps betrayal. What’s wrong? Should I pull over? Is my car safe to drive? What does the Service Engine Soon light actually mean?

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A Service Engine Soon light will appear when your cars onboard diagnostic system (the ECM) detects a potential problem. Its purpose is to alert you of any minor issues before they become major concerns. Common causes include needing an oil change, or a new cabin filter or air filter.


Great news, because in this article, we’ll cover everything you need to know about this helpful little light. Including how it differs from a check engine light and what causes it to come on. We’ll also examine what to do if one shows up, and how to diagnose the issue yourself using an OBD II sensor.


Let’s get started.


What’s the Difference Between a Service Engine Soon Light and a Check Engine Light?

Like most things nowadays, a lot of your car is controlled by computers.  Everything that happens under the hood is managed by the ECM (Electronic Control Module).  A Service Engine Soon light typically illuminates in response to a minor engine/electrical issue. Some of these include a loose gas cap, low fluid levels, or a problem with the emissions system.


Should anything not be operating as intended, such as something running too fast, too slow, too hot, or too cold. That data gets relayed through the ECM.  If certain conditions are met that could lead to your engine having issues, it will prompt the Service Engine Soon light to appear.


A “Check Engine” light, in comparison, typically appears in response to a more immediate concern. Some of these include the mass airflow sensor needing to be replaced. An issue with the catalytic converter, or that not all of the cylinders are firing correctly. Think of the Service Engine Soon light as a mini Check Engine light. The two are so close, in fact, that some manufacturers combine them.


For this reason, we’ll cover both as one and the same, since both are an alert letting you know there’s a problem that needs your attention.  Let’s now look at a few of the most common causes why a Service Engine Soon or Check Engine light might come on.


Common Causes of a Service Engine Soon and Check Engine Light


As we’ve mentioned, a Service Engine Soon or Check Engine light will come on when there’s an issue. Some of these range from small and quick to fix, to severe problems that can be catastrophic to your engine.


Let’s look at a few of the most common:


  • Loose/Faulty Gas Cap

The first thing to do when a Service Engine light comes on is to check the gas cap. When your gas cap is loose or has a crack, fuel vapors can seep out, causing the ECM to think there’s an issue. This can cause a drop in mileage and increase emissions, so it’s best to handle it right away.   Whether you didn’t screw it on properly last time you filled up, or if it’s broken, the fix is simple.


First, with the engine off and having at least a half tank of gas, unscrew the cap and then put it back on.  It usually requires a few recycles before the light will go off, meaning you’ll need to drive the car a few times.  If after 30-min to an hour of driving go by and it doesn’t turn off, you should consider buying a new one for around $10 at your nearest auto parts store.


If the indicator light doesn’t turn off shortly after that, there must be another issue.


  • Scheduled Service/Low Fluid Levels

While being a week or two past a scheduled oil change might not set off an alert, if you’re critically low on oil, it might.  The same goes for other maintenance items, like an air filter, which, if it’s unable to do its job, can throw the whole system off balance.


If your Check Engine light appears due to a maintenance item, depending on the make of your car, it will likely accompany another indicator. Such as an oil light, a low coolant light, or a tire pressure light.


  • Oxygen Sensor Failure

Your engine is designed to operate at an optimal level, using sensors to keep an eye on things.  Some of these include the mass airflow sensor, engine speed sensor, spark knock sensor, and coolant sensor. If something happens that jeopardizes the ability of your engine to maintain this level, things can go downhill quickly.


Then there’s the O2 sensor, which monitors the level of oxygen in the exhaust as it comes out of the engine. If there’s not enough, it’s running rich, too much, and it’s running lean.


When there’s an issue with your O2 sensor, it will likely trigger a Service Engine Soon light. Why? Because it can’t properly maintain the correct air/fuel mixture in the combustion chambers. Alongside this indicator, you’ll also likely notice reduced fuel economy, misfiring, or a rough idle.


  • New Spark Plugs Needed

Sparks plugs are what ignite the air/fuel mixture in the combustion chambers of your engine.  Signs you need new ones include reduced mileage, a lack of acceleration, misfiring, hard starts, a rough idle, or (you guessed it) a Service Engine light.


While older copper spark plugs were only good for about 30,000 miles, new platinum or iridium tipped one can last 100,000 miles or more.


If you ignore them for too long, your engine may fail to run at all, which is a solid reason to replace them at the first signs of an issue.  Especially since they only cost about $10 each, and with labor, you should be able to replace them all at a local shop for less than $250.


  • Mass Airflow Sensor Failure

The mass airflow sensor instructs the ECM on how much fuel it needs based on the amount of air it is receiving.  Signs of a problem include the engine having a hard time starting, stalling, or hesitating at idle or while accelerating.  If these issues persist, your car will alert you with a Service Engine Soon light.


While you can drive with a faulty mass airflow sensor, you shouldn’t do so for too long.  Ignoring it can lead to costly repairs, such as ruining your catalytic converter or O2 sensors.  Costs (including labor) should range between $200 to $400, depending on if you go with aftermarket parts or OEM.


  • Faulty Catalytic Converter

A combustion engine works by taking gasoline or diesel (made from petroleum) and igniting it to produce power. With pure petro, the byproduct would only be carbon dioxide and water. But these fuels contain as much as 150 added chemicals, meaning more pollutants get created during the process.


A catalytic converter is in charge of converting these harmful gases into something less taxing on the environment. The most common reasons they might have problems are due to the other items on this list failing first.  If your Service Engine Light comes on and it coincides with a temperature gauge in the red, reduced performance, or a decrease in mileage. It might be time to get your catalytic converter checked.


Replacing one on a newer vehicle can run more than $2,000, which is a great reason to stay on top of engine issues at the first signs of a problem.


Now that you know a few of the most common causes of a Service Engine Soon light.  Let’s look at what to do if you have one show up.


What Should I do If My Service Engine Soon or Check Engine Light Comes On?


  • Flashing/Solid/Color of the Light

As we mentioned earlier, some cars have a Service Engine Soon light as well as a Check Engine light, while others only have one.  The severity of the issue is usually represented by whether it is solid or flashing.  Some manufacturers take it a step further and color code the indicator.


Solid, or yellow, typically mean it’s a minor issue that should be handled in a reasonable amount of time.  If it’s flashing, or red, this suggests a significant problem, meaning you should pull your car over when able and immediately turn the engine off.  To be on the safe side, tow it to a shop rather than drive it.


  • Pay a Shop to Diagnose the Issue

Clearly, there’s always someone willing to take your money when it comes to car repairs.  This typically involves setting up a time to drop it off and allowing as much as half a day for them to complete the diagnostic.  If you don’t have the luxury of a second vehicle, this means having a friend or family member follow you to drop it off and pick it up, or taking an Uber both ways.


Expect to pay as much as $100, not including if you paid for a ride. The good thing is that you’ll know the exact reason the light came on.  Some shops may wave the diagnostic fee if you’re willing to agree to let them handle any repairs.  Just be sure to go through a shop you trust, the last thing you want is to end up getting overcharged for something minor.


  • Bring Your Car to an Auto Parts Store

While it’s not as thorough as bringing it to a shop, you can also bring it to the nearest auto parts store.  Most offer a free service where they’ll use an OBD II scanner and let you know what it says.  Not only is this faster than bringing it to a shop, but if it turns out to be something minor, you might be able to resolve it yourself.


Now that you know what to do when a Service Engine Soon light comes on.  Let’s look at another option, buying an OBD II scanner and diagnosing it yourself.


What is an OBD II Scanner and Why Should I Get One if I Have a Check Engine or Service Engine Soon Light On?


OBD stands for On-Board Diagnostic, and you can get one either at your local auto parts store or online for anywhere between $30 to $200. Regardless of the price you pay, each will do the same thing. Though, some might have a more detailed LCD screen to make it easier to read.


Does having an OBD II scanner suggest you don’t have to take your car to a shop? No, it just means you can diagnose the source of the issue yourself. If the fix is something small, like a faulty ignition coil, bad spark plugs, or gas cap. You won’t have to spend money for a professional to do something you can quickly find instructions for online.


First, locate the Diagnostic Link Connector (DLC).  You’ll usually find this under the drivers-side dash near the steering column, it’s a triangle-shaped port with 16 pins. If you can’t seem to find it, consult your manual for its exact location. Then, plug in the OBD II scanner with the ignition in the accessory position (do not start the vehicle).  You’ll then be prompted to enter a few details about your car, such as the Vehicle Identification Number (VIN), as well as the make, model, and engine type.


Depending on the scanner you purchase, the process may differ. Regardless, you’ll need to navigate to the “codes” menu. You should then see a few codes, each starting with a letter. (P) for the powertrain, (B) for the body, (U) for undefined, and (C) for the chassis.  Following the letter will be a series of numbers indicating the make of the vehicle, the subsystem affected, and what the problem is.


It can be confusing if you’ve never used one before.  Thankfully, you can search for any specific code online to find out what it means.  Here are a few of the most common.


Feeling Better About That Service Engine Soon Light?

Nobody likes finding out that there’s an issue with their car.  Thankfully, a Service Engine Soon or Check Engine light doesn’t mean it’s the end of the world.  It’s just a handy way of knowing there’s a problem that requires your attention.  By finding out what the issue is early, and resolving it, you reduce the chances of it becoming a major concern.  Saving time, money, and getting you back on the road to live another day.




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