Why Is My Check Engine Light On? The 5 Most Common Causes

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Check Engine Light
Check Engine Light/Image Credit: BDoty

Is your “service engine soon” or “check engine” light illuminated? Does it come on briefly and then disappear? Does it blink or flash and then go off, or come back on and remain on for extended periods of time? Your vehicle could be telling you that something is very wrong.

However, it’s also possible that the problem is minor, but you’ll need to address it sooner or later. The longer you wait, the more likely the issue will become more serious and/or cost you more money in fuel efficiency. Plus, if you reside in an area where vehicle emission testing is mandated, then it is only a matter of time until your service engine light woes must be addressed.

What to Do First

The first thing you should notice about your check engine light is whether it’s flashing or just simply illuminated. A flashing light indicates there’s a serious problem and if you continue driving the vehicle, there is a good chance that the engine will be severely damaged. If that’s the case, get the vehicle to a repair shop immediately and avoid driving it altogether.

If the light is steady and not flashing, you have more time to evaluate things yourself. In this case, your best bet is to take the vehicle to an auto parts store and have them scan the “On-Board Diagnostics” system for the trouble code. Since 1996, every car, truck, SUV, and van sold in the United States has come with an OBD-II port, usually somewhere under the dash.

Innova OBD2 Code Reader/Image Credit: Auto Zone

Most auto parts stores have OBD-II scanners on-hand that they can plug into your car’s OBD-II port and the vehicle’s computer will give them a code to indicate the problem. Almost any auto parts store will scan the code for free and print it off for you. If you’re the DIY type, an OBD-II scanner is a great addition to your toolbox as well.

Once you have the code, these are some of the most common issues you’re likely to come across:

5. Cylinder Misfire

One likely cause of your glowing service engine light is a cylinder misfire. Designated with trouble OBD codes 300 through 312, these types of codes usually occur around the 100,000-mile mark and eventually involve periodic replacement of all the ignition components or a good preemptive tune-up (recommended). Spark plugs, ignition coils, and ignition coil boots can all be listed as common causes of these ignition codes.

4. Intake Air Temperature (IAT)

The intake air temperature (IAT) code differs from other codes on our list in that it is typically caused by human error. When the air box is disturbed, for purposes of air filter replacement, the IAT connector is often left disconnected, causing an inadvertent code to be set and a service engine light to be illuminated. If the connector is plugged in and still causing a code, then you may have a faulty sensor or a problem with the wiring/connector itself.

3. Oxygen Sensor Heater Circuit

Late model oxygen sensor heater circuits are yet another common cause of the annoying service engine lamp. The good news is that some manufacturers have “smartened up” and begun to utilize fuses in an effort to spare costly sensor heater elements (not that it always works out that way).

2. Exhaust Gas Recirculation (EGR) Valve

This is the most valuable tip of the article: Just because the code description has the words “EGR valve” in it, doesn’t necessarily mean that replacement of the exhaust gas recirculation (EGR) valve will be required. In fact, it seldom is the cause. Try faulty vacuum or exhaust lines and hoses, vacuum solenoids, or clogged EGR passages in the intake.

1. Loose Gas Cap

Easily taking the top spot is the evaporative emissions system leak (large). This code can be caused by failure to tighten the fuel filler cap tightly. Talk about an easy fix!

Unless you’re a world-class mechanic, you won’t be able to solve every problem yourself. However, knowing what to do when a warning light comes on is crucial to staying safe on the road and avoiding costly repairs. Is your current vehicle giving you more trouble than it’s worth? Consider trading it in and/or buying a new one here!