Retrieve a saved quote

You May Also Like

Main Content
Check Engine Light On? What It Means and How to Fix It? | Allstate

Check Engine Light On? What It Means and How to Fix It

December 16, 2019 If you've been driving for a while, the chances are you've seen the little 'check engine' light appear on your dashboard. But what does it mean? Should you stop driving immediately? Can you drive to a repair shop? Do you need a tow truck? Is the problem identifiable? Can you… Allstate
dashboard check engine light.

If you’ve been driving for a while, the chances are you’ve seen the little ‘check engine’ light appear on your dashboard. But what does it mean? Should you stop driving immediately? Can you drive to a repair shop? Do you need a tow truck? Is the problem identifiable? Can you fix it yourself?

Many situations can trigger the check engine light to come on and it can be very confusing. To help you better understand your check engine light, auto maintenance and repair specialist The Humble Mechanic explains what it could mean and what actions you can take.

Get a quick, personalized insurance quote today.

Select coverage Type

Watch more videos by The Humble Mechanic. Follow him on his blog, Twitter and Facebook.

Originally published May 2016.

Hey everybody, it’s Charles from and today we’re talking about the dreaded “check engine” light.

When the check engine light comes on in our car, it can be a pretty scary thing. Then little amber light will pop on in our dash to let us know something’s wrong, but it doesn’t really tell us much more than that. So today I’m gonna break down what the check engine light means, what to do when it comes on and how we can make it go away.

The check engine light in our cars is there as part of a warning system that lets us know that, hey, there’s an issue with the powertrain of our vehicles. The powertrain can be our engine; it can be our transmission; if we have all-wheel-drive it can include the all-wheel-drive component. Basically, all the things that we need to make our car actually move are considered part of the power train. So, when that check engine light comes on, those are the systems that were initially focused on.

Now there’s a ton of reasons that our check engine light can come on. It can be a simple problem, like maybe the last time I put fuel in my car, I didn’t tighten the gas cap all the way. It can also be a really, really in-depth problem with sensors or wiring or really anything in between. Every time we drive our vehicle, even sometimes when the key is simply turned on, our car is running a series of tests. These tests are run to make sure that things in our car are functioning properly, not only for emissions controls, but for drivability issues as well.

Now, when the car’s engine computer sees a problem–sees a failure of one of these tests–it turns the light on to let us know that we’ve got something going on and we need to get it taken care of. There’s basically two types of monitoring that our engine computer does: there’s continuous monitoring and non-continuous monitoring.

Alright, let’s look at continuous monitoring first systems in our vehicles engine that are continuously monitored are basically being checked anytime the engine is running. Some really common ones are engine timing concerns and misfire concerns. Those are monitored any time the engine is running and that’s mostly because if something goes wrong with that, there can be catastrophic failure pretty quickly.

Now, when it comes to non-continuous monitoring, our ECM or engine computer, sometimes also called the PCM for powertrain control module, will run these tests when the conditions are right. For example, when your engine computer runs the catalytic converter test it has to run that test when the catalytic converter is at the proper temperature. If it tried to run that test when the catalytic converter wasn’t, it would fail. These are really how most things are monitored in your car. These can also be called two-trip monitors because generally it takes to failures before the check engine light will actually come on in your car.

Alright, so let’s break down exactly what this check engine light can mean. If your check engine light comes on, it’s not a big surprise that your car is telling you: Hey, I got something wrong, you need to take me and get me fixed. But it doesn’t really tell you much more than that. You’re going to need some kind of diagnostic tool to access your engine’s computer. If your car’s ‘96 or newer, you’re on the OBD-II platform, which means that the automotive industry has set standards for certain things like where your data link connector is and what information has to be given when a check engine light is stored. What that means is, you don’t have to take it to a specific dealer for the brand to get it inspected. You can take it anywhere that has these OBD-II diagnostic capabilities and have the check engine light checked.

When we hook this diagnostic tool up to your car and go into the engine computer, it can give us information like fault codes. Fault codes give us the system that has a failure, but it may not tell us exactly what the problem is. For example, a fault code might look something like this: P0300. The first character will point to the area of the car. The P stands for Powertrain. Faults can also begin with B for body, a C for chassis or U for undefined.

A lot of times when you see a U code, that’s a manufacturer-specific code and that’s one of the cases where only the dealership might be able to have that information. The second digit will tell you the code is generic, meaning the same on all ODB cars, or manufacturer-specific. Zero indicates a generic code and a 1 would indicate a manufacturer-specific code.

So, in our P0300 code we know that we had a powertrain code and then the next digit is a zero so we know that this is a generic code that would apply basically to all cars that are OBD-II.

Now, the third character is where it starts to get interesting, because this gives us the subsystem of the fault. 1 is engine management; think air and fuel system. 2 is the injector circuit it can also be the fuel or air system. 3, like our example, is engine misfires. 4 is emissions controls. The catalytic converter example I just mentioned, that code typically would be a P0420, indicating that there’s a problem with the catalytic converter. 5, we have vehicle speed and idle control. 6, computer and output circuits. 7 and 8 are both transmission, and then 9 and 0 are reserved for the SAE. SAE is the Society of Automotive Engineering. Now, these are the folks that put together basically all of these standards for the automotive industry.

Now, I’ve given you guys a few different examples of engine fault. That P0300 fault, we know that’s a powertrain fault that is generic across the OBD-II spectrum; 3 is an ignition problem. Now the last two digits are going to be the identifiers. So, if this were a four-cylinder car and we had a P0300 and a P0301 appears, we would have a cylinder one misfire. If it was a P0302, it would be cylinder two, and so on.

So what do we do with our check engine light comes on? Well, there’s three basic states to the check engine light. There’s the check engine light is off. That means everyone is happy and that is ideally what we want to have going on. We have our check engine light where it is simply illuminated. That means our car has a problem and we do need to get it checked as soon as we possibly can. We also have on and off flashing. This points to an engine misfire. This can be a very serious thing. We want to make sure that we’re not driving a vehicle with flashing check engine light — it can do damage to our catalytic converter. Getting your car towed is highly recommended if it has a flashing check engine light.

Alright so what are we doing this check engine light comes on? Well, we have several options. We can take it to our local dealership and have them do a full diagnosis. Dealerships know cars very well; they know their brands very well; so, that’s a great option. There’s also a ton of really great independent shops that specialize in maybe the brand of your car, and you can even take it there for a full diagnosis. There’s also a third option and that’s purchasing our own OBD-II scanner. Now maybe something like this OBD-II scanner and you can even use something like your cell phone — pair the two — and we can look at what’s going on with our own car.

With each of these options come some pros and some cons. If we take our car somewhere — to the dealership or to an independent shop – odds are we’re going to charged the diagnostic fee. But the good thing is, they should have all the information and all the equipment to properly diagnose what’s going on with your car’s check engine light. Using our own OBD-II scanner like this one may save money on the diagnostic process, but we might not have all the repair and all the diagnostic information. We may also lack the test equipment to properly pinpoint exactly what’s going on with our car.

Before you decide to DIY or not to DIY, I highly recommend you check out another video that I did called 7 Simple Car DIY Mistakes to Avoid. That will give you more information to help me decide do you want to do a DIY, or is this something better left to the professionals.

Now we understand a little bit more about what this check engine light means, let’s talk about how to get rid of this pesky little light. The best course of action, of course, is to get your car fixed — whether that’s a DIY or whether you take it to a professional. The best thing obviously to do is to get whatever problem your car has completely taken care of.

Again, this could be as simple as maybe tightening the gas cap all the way. It can also be as complex as maybe needing an entire fuel system in our car. But after these repairs, were also going to need to clear the check engine light and let the car run all of its tests again to make sure that we don’t have any secondary issues going on.

For the most part, even the DIY OBD connectors can clear the check engine light once we get those codes cleared, the car’s going to need to re-run all those tests. In order for all of those tests to run, the car has to complete what’s called a drive cycle. This varies tremendously from manufacturer to manufacturer and it basically puts your car in several different driving situations – be it cold; be it hot; maybe cruising on the highway; acceleration and deceleration…in order to run all the test to test all the components your ECM or your engine computer does monitor.

All of these tests that run, whether it’s on a drive cycle, whether it’s forced with a diagnostic tool, is called setting readiness. The readiness monitor is seen by the engine computer as an eight-digit set of zeros and ones, and this can tell a technician what test run and passed and what tests haven’t passed yet.

In several states that have emissions inspections, when your car set up to the states computer that’s what they’re looking for. They’re looking for all those readiness monitors to be set; all those tests to be passed. Now, it’s not generally recommended to just go in and clear the faults out. As a professional technician, I will do this from time to time but it’s not just to erase the code so the light’s not on, it’s generally part of the diagnostic process.

See, when you delete these faults it also deletes very important engine information. Some faults will be stored with parameters to them: Was the engine hot? Was the engine cold? What RPM was the fault stored? And these things can be vital for a technician in order to properly diagnose what’s going on with the car. When you delete the fault, it also erases that information and that can make check engine light diagnosis even more difficult than it already can be. So if you have a problem with your car, I generally don’t recommend just simply clear the fault.

Alright guys I’m going to go ahead and wrap it up there. If you have any questions comments or check engine light stories, feel free to post it down in the comment section below. If you want to check out more of my videos, head over to, you can see them all. You can also follow me on Facebook, Twitter, Instagram and YouTube. Remember guys, when your check engine light comes on, don’t necessarily panic, but I also really don’t recommend waiting too long to get it fixed, because that can actually cause more problems. Alright guys, thanks for watching and I will see you next time.

Sign up for our email newsletter.

Get more tips and tricks like this delivered straight to your inbox each month when you sign up for our newsletter.
You May Also Like