My Check Engine Light Is Coming On

Reader Question:
My car check engine light has been on for two days. What should I do? Will I damage my car if I continue to drive it?

Thanks, Shannon


Hi there Shannon,

I get questions on check engine lights at least once a week. The dashboard on most cars is completely cluttered with warning lights and indicators. It can be really confusing for many drivers.

First, you have to figure out what car function the indicator is warning you about, then comes the fun part of figuring out what to do with that information.

Your dashboard lights are color-coordinated according to the severity of the warning (i.e., yellow is caution, red is a serious warning). Each time you start the car, all of the lights and indicators should come on.

The car’s computer checks each system, and then that light is turned off as the computer verifies the part is working properly. Therefore, you only need to be concerned with a light that is still on several minutes after the car has been started or that comes on while driving the car.

A problem causing a check engine light can be tricky to diagnose and repair, so knowing what to look for and what steps to take can save you and your mechanic diagnostic time and hopefully save you cash. Treat a check engine light, and all dash lights, like traffic signals.

A Yellow Warning light,

On most cars, a yellow Service Engine Soon light is telling you to “proceed with caution.” Your car’s computer system has indicated a potential problem and is alerting the driver that something isn’t right.

As previously mentioned, this computer system resets and re-tests all sensors and indicators each time the car is started.

The on-board computer can make minor corrections to help improve engine efficiency, but if the light continues to come on after you have been driving long distances and or multiple starts and stops, it is telling you that you need to consult a technical manual and/or see your mechanic.

If the light comes on while you are driving but doesn’t come back on tomorrow, the computer system probably corrected the problem without any further intervention.

If you are driving your car and a yellow light comes on, ask yourself these questions:

1. Did you have any problems (such as slower than normal to start) when starting the car?

2. Has there been any recent changes in fuel mileage/fuel efficiency?

3. What is the current running condition of the car? Is the car’s overall performance normal? Is the engine misfiring?

4. Have you noticed any new noises from under the hood? Turn off the radio and listen closely as you drive the car.

5. Do you see visible smoke from the tail-pipe or from under the hood? Have you noticed any odors, such as a rotten egg smell?

Common problems that may cause a yellow Service Engine Soon light to come on, but will probably require an experienced technician or a really good auto repair manual to diagnose are:

  • computer-related problems, such as faulty sensors
  • anti-lock brake systems
  • safety restraint systems, such as air bags
  • emissions components

Red Warning Lights

A Red Warning light indicates an immediate danger to your car. Immediately pull over and turn off the car. Do your best to assess the problem.

Problems that normally cause red warning lights are low oil pressure, engine over-heating, brake failure, transmission over-heating, or low battery voltage.

Whatever caused the red light to come on can be a safety issue (i.e., brakes), so proceed with caution as you maneuver to stop the vehicle.

Unfortunately, when the red light is comes on, the problem is already occurring–this is NOT a pre-warning. It can be a matter of seconds before permanent engine damage occurs.

Some common problems when a red light comes on are:

1. Missing or damaged fan belt

2. Low anti-freeze, engine oil, and transmission fluid levels

3. Low brake fluid

4. Bad battery or bad alternator that is not charging the battery

To decode a check engine light, you really need a quality repair manual like those provided at ALLDATA auto repair manuals. These manuals provide troubleshooting procedures, recall Information and Technical Service Bulletins.

When you experience a yellow or red warning light, make a note of which indicator it is and the wording/picture on the light that has been coming on. Murphy’s Law loves car troubles and that the light may not be on when you take it in for your mechanic to see.

With some cars, the “engine” indicator is the more serious red light, whereas others use “check engine light,” which is a yellow caution light. If you inadvertently lead your mechanic to believe you are seeing a yellow “check engine light,” and your car is indicating a red “engine” danger light, this could be a costly mistake for both of you.

Your car could have a serious overheating problem, and the mechanic is assuming you are seeing a less serious caution light. The mechanic then might give you the “green light” (no pun intended) to keep driving the car till he can get to it later in the week.

When communicating with your mechanic about the lights that have been coming on, start the car, and point to the light to make sure you are communication the correct problem.

Dash warning lights can be a pain and hard to diagnose, especially when they are coming on and off! Sometimes the light must be on for the mechanic to perform the diagnostic test.

If the light is not on when you bring the car to the shop, you might be wasting your time and the mechanic’s, and your money.

NOTE: Keep a dash light and running condition log in your car to help the mechanic locate the source of the problem.

One of my customers keeps such an accurate log of her cars’ complaints, that I have been able to diagnose a problem correctly during my initial test drive with the help of her log.

This effort on her part saved me time, and she saved money on a diagnostic charge. This customer can also hold me to my warranty time period, because she had this particular complaint written in her log book.

Austin Davis

Posted in: Check Engine Lights

About the Author:

Austin Davis, consumer car repair advocate. "Hi there! I love to help people solve their car repair problems and I hope my site was helpful to you today. Thank you for stopping by."
  • Austin Davis

    Awesome, thanks for commenting Nicola glad it was helpful to you.

  • Nicola

    This is good information. I am not very mechanical at all and even I was able to understand what is happening with my car and why the check engine light has come on. Thank you.