The check engine light is a commonly misunderstood device on a car. The lights go on and the driver freaks out, thinking the vehicle is about to explode. Understanding what it's supposed to do will also help you anticipate what things are unexpectedly causing it to show up and what you can do about it. The check engine light is a signal that the onboard diagnostics (or OBD II) has detected a malfunction in the vehicle's exhaust, ignition, fuel, or emissions systems. It could be caused by something as simple as a loose gas cap, or a faulty oxygen sensor, or a faulty spark plug, or something as serious as a faulty catalytic converter. Because many of the problems identified are related to the emissions system, they are not that simple and do not necessarily render a car inoperable. To understand how it relates to emissions, you need to understand the main purpose of the on-board computer. Although it monitors many engine functions, its primary purpose is to provide the catalytic converter with an air/fuel ratio of 14.7. This is not an ideal ratio for performance, but it is the air/fuel ratio at which the catalytic converter works best. Anything interfering with this will result in an illuminated check engine light. When the check engine light comes on, the computer sets codes in its memory that help a technician diagnose the problem.
The check engine light is a commonly misunderstood device on a car. The lights go on and the driver freaks out, thinking the vehicle is about to explode. Understanding what it's supposed to do will also help you anticipate what things are unexpectedly causing it to show up and what you can do about it.
The check engine light is a signal that the onboard diagnostics (or OBD II) has detected a malfunction in the vehicle's exhaust, ignition, fuel, or emissions systems. It could be caused by something as simple as a loose gas cap, or a faulty oxygen sensor, or a faulty spark plug, or something as serious as a faulty catalytic converter. Because many of the problems identified are related to the emissions system, they are not that simple and do not necessarily render a car inoperable.
To understand how it relates to emissions, you need to understand the main purpose of the on-board computer. Although it monitors many engine functions, its primary purpose is to provide the catalytic converter with an air/fuel ratio of 14.7. This is not an ideal ratio for performance, but it is the air/fuel ratio at which the catalytic converter works best. Anything interfering with this will result in an illuminated check engine light. When the check engine light comes on, the computer sets codes in its memory that help a technician diagnose the problem.
The most common causes - gas cap and oxygen sensor
The two most common reasons for a check engine light to come on are a loose gas cap or a problem with an oxygen sensor.
The oxygen sensor detects the oxygen content in the afterburning emissions and looks for values within a certain range that tells the computer everything is working properly - the engine is burning the fuel the right way and the emissions are in balance. Oxygen sensors detect air/fuel ratios in the exhaust and generate voltage signals based on what they detect. These sensors are actually voltage generators that operate on a scale of 0 to 1,000 millivolts. A reading above 500 millivolts indicates the air/fuel ratio is rich, meaning more fuel and less air. A reading below 500 millivolts indicates a lean condition, which means more air and less fuel.
The oxygen sensor sends these voltage signals to the on-board computer, which can adjust the amount of fuel delivered based on what the sensor says. The computer's goal here is to keep the air/fuel ratio at an optimal level for the best emissions from the vehicle. So to keep the check engine light from coming on, the computer sees the voltage signal from the oxygen sensor and either adds fuel or subtracts fuel to keep the voltage reading close to 500 millivolts, which is 14.7 to 1 air/fuel relationship happened. If the computer cannot correct the ratio, the check engine light will illuminate and the appropriate codes will be set in the computer.
If the oxygen sensor is wrong based on what it "sees" in the emissions, it can tell the computer to adjust certain operational things (like the timing) to see if it can fix the problem. If the problem isn't correct, the check engine light will come on, which is the computer's way of telling you "I can't fix this myself". They take it to a mechanic, they plug a code reader into the car's computer, and the computer tells the machine what it sees. This gives the mechanic some strong clues as to what to look for in order to fix a particular problem.
If the gas cap is loose and the reason why the check engine light comes on, you just need to tighten the cap and after a short time while driving, the light will go out.
check engine light on? Check your fuel
Believe it or not, your fuel can be a major trigger for conditions that lead to your check engine light tripping in a number of ways.
First, let's talk about ethanol in gasoline, which can be a check engine light trigger. This can happen when the computer expects to see one thing and it's still seeing another; This will cause the computer to turn on the warning light in response. Non-flex fuel vehicles are not designed to operate on ethanol concentrations greater than 15%. Flex-fuel vehicles can use any concentration of ethanol up to 85%, known as E85. Ethanol has less energy than gasoline; Therefore, more fuel is required to achieve the same performance. The fuel injectors in non-flex vehicles differ from those in a flex-fuel vehicle. Flex fuel injectors coupled with the computer allow for greater fuel flow. If higher concentrations of ethanol are present in your non-flex vehicle's fuel tank, it will cause performance and drivability issues and the check engine light will illuminate.
Put another way, non-flex fuel vehicles (i.e. probably your car) are designed to run on 15% or less ethanol in the fuel. They are not designed to run on, say, 85% ethanol (E85) fuel - and by that we mean that the timing and injection in the engine are set to work properly at lower concentrations of ethanol. Why is this important if your car isn't set up that way? If you put E85 in a vehicle where the computer expects 15% ethanol or less, the oxygen content in the emission that the sensor reads will be dramatically different. This confuses the computer, which doesn't expect the oxygen levels to be that high. The computer thinks it's not delivering enough fuel and starts injecting too much fuel than the engine really needs, while also playing with the timing by changing how often and when the spark plug fires. In most normal cars (i.e. non-flex fuel) it wouldn't even run in a situation like this.
Beyond the simple ethanol percentage, gasoline that lacks the proper octane rating that your vehicle is designed for can cause the engine to start. The computer system monitors a knock sensor in the engine block, which detects the ringing or rattling noise caused by low octane pre-ignition. The computer will then delay the timing until the noise stops. If it cannot stop the ping, it will illuminate the check engine light and set diagnostic codes in the computer's memory.
The third area of concern is unscrupulous service station owners who could substitute methanol for ethanol. Methanol alcohol is cheaper than ethanol or gasoline, and adding methanol to the gasoline storage tank to rake in extra bucks can adversely affect your computer's ability to regulate the air/fuel ratio. Methanol contains more oxygen, resulting in higher oxygen concentrations in the exhaust stream. The computer detects this lean air/fuel ratio from the oxygen sensor readings and adds additional fuel to maintain the 14.7 to 1 air/fuel ratio discussed previously. This will ruin your good fuel economy and if the computer can't correct this by adding extra fuel to correct the reading, the check engine light would come on, alerting you to a problem.
So whether it's too much ethanol in the fuel or methanol that shouldn't be there, in these two highly plausible scenarios the computer sees more oxygen in the exhaust than it thinks it should. But it's not so far out of balance that the engine can't run. It just doesn't do nearly as well with 21% ethanol or 5% ethanol + 7% methanol. So the computer starts playing with the timing and fuel injection and you end up using more fuel than you should (lower mileage). And if it can't fix the problem that way, it turns on your check engine light.
Finally, there are many other reasons why a check engine light comes on. If the check engine light comes on while driving, but you don't notice any change in how the engine runs, there is most likely an issue with the exhaust system. You should get this checked out ASAP, but it's not something that's going to stop you. If the check engine light comes on and you notice a negative change in engine operation, it needs to be checked immediately. When engine failure occurs, continued vehicle operation can result in a damaged or destroyed catalytic converter, a very expensive proposition.
You might be interested in these other posts:
- Wait! Should You Buy a Used Car? Check out these signs you shouldn't.
- Beware of certain types of ethanol gas treatment
- Falling gas prices are affecting ethanol for consumers
This post was published on October 30, 2013 and updated on August 2, 2022.
Can ethanol gas cause check engine light? ›
If higher concentrations of ethanol are present in the fuel tank of your non-flex vehicle, performance and drivability issues will result and in turn, the check engine light will illuminate.How long until check engine light resets after loose gas cap? ›
Loose Fuel Cap
If you find that the cap has damage, you can replace it for around $15. After that, you can start driving around and check if it resets after 50 to 100 miles.
There are a couple of simple solutions if your check engine light came on after getting gas. A loose gas cap is one of the most common reasons for the check engine light to go on. Check that your gas cap is screwed on securely and that it's in good condition.How do you reset the check engine light? ›
Simply place your key into the ignition and turn it on for 1-2 seconds, then turn it off for 1-2 seconds. Repeat this step three or four times. If the check engine light remains after the reset, you might still have a problem with your vehicle.What happens if you accidentally use ethanol gas? ›
Continually filling a standard gasoline car with this high-ethanol fuel can result in serious fuel system corrosion. Plus, when you accidentally put E85 fuel in a car, it may void your engine warranty. That means you could be on the hook for the repairs, even if your car is brand new.Will gas with ethanol hurt my car? ›
The EPA and an academic study have said that fuel containing 15% ethanol is safe for cars, trucks and sport-utility vehicles made in 2001 or later — which make up more than 90% of the vehicles on U.S. roads. Many car manufacturers have okayed the use of E15 fuel in their vehicles made in the past 10 years.How many miles do you have to drive to reset engine light? ›
To make sure the check engine light does not reappear, it's recommended that you drive your car 30 to 100 miles. This enables the vehicle's “Drive Cycle” to reset, as the various sensors need time to recalibrate.How do you turn off the check engine light without a scanner? ›
To reset a check engine light without a scanner, disconnect the negative battery cable and wait a few moments. This will reset the diagnostic system in your vehicle, and if the underlying issue has indeed been resolved, the light should go off immediately.How do you reset a gas cap sensor? ›
The Gas Cap Trick
Simply remove your gas cap and put it back in place. Turn your car on, and the engine light may already be reset. If not, it might take a few cycles of turning your car off and on to make it reset. In some cases, it helps to drive around for a few miles, too.
The emissions/exhaust system is the most likely culprit for many check engine lights being illuminated. There could be an exhaust leak or a problem with the catalytic converter. The mass airflow sensor (MAF) is another sensor that is easily tripped or can fail.
Can a check engine light go away by itself? ›
A check engine light will shut itself off if the condition that caused it is remedied. So, if your converter is marginal, and you did a lot of stop-and-go driving, which creates high demand for the converter, that may have turned on the check engine light.Why is my engine light on but nothing seems wrong? ›
If nothing seems to be acting strange, then it is probably safe to drive it until you can get it into a mechanic. Sometimes the light may come on after fueling if the gas cap is a little loose. Or it could mean your catalytic converter needs to be inspected.Do you have to manually reset check engine light? ›
“So what” you might think, “I'll clear the check engine light and then drive around so the monitors will run, then I'll go for a smog test.” Well, if nothing is wrong with the car, this will work. However, if the monitor runs and passes, the check engine light will turn off by itself. No reset is required.Does ethanol mess up your engine? ›
Ethanol can cause several types of damage to the engine in your vehicle. Your vehicle's fuel intake components can be damaged. In addition, ethanol can cause damage to the fuel pump in your vehicle. These repairs and replacements can range from a few hundred dollars to more than $1,000 to take care of properly.How do you get rid of ethanol gas? ›
Removing ethanol from gasoline is easier than it sounds. Ethanol is more soluble in water than it is in gasoline. Therefore, if you add water to the gasoline and vigorously shake it, the ethanol will attach itself to the water.Can you remove ethanol from gas? ›
The ethanol itself is trucked in by big rig or rail car (since it can't go through the same pipelines as other parts of the fuel). Once it is blended into gasoline, there's no feasible way to separate ethanol from the gasoline.How long does it take for ethanol to separate from gas? ›
“In a small engine fuel tank in a constantly high-temperature, high-humidity environment, it takes three months or longer for E10 and other ethanol blends to take up enough water for phase separation,” says NREL.What cars can use high ethanol gas? ›
|Chevrolet Silverado 2WD 2023||Ethanol (E85)|
|Chevrolet Silverado 4WD 2023||Ethanol (E85)|
|Ford F150 Pickup 2WD FFV 2023||Ethanol (E85)|
|Ford F150 2WD FFV 2023||Ethanol (E85)|
The disadvantages of ethanol and other biofuels include the use of farmland for industrial corn and soy growth, rather than for food crops. Also, biofuels aren't meant for all vehicles, especially older vehicles. There is some resistance from the automotive industry when it comes to adding biofuels to the market.Is it OK to drive with engine light on? ›
Can you drive a car with the engine light on? It's okay to drive for a few miles, but be sure to schedule an inspection of the engine as soon as possible. If the check engine light comes on while you're driving, don't panic! Pay attention and see if the car is driving any differently than normal.
Where is the fuse for check engine light? ›
The engine compartment fuse box is under the driver side leaf screen in the engine compartment.Will disconnecting battery reset check engine light? ›
3) Disconnect the Battery then Reconnect
Ideally, this will clear the data, turning off the light. You can then reconnect the battery and turn the ignition on. After about a minute, the check engine light code should be off.
If you see a P0442 evaporative emission system leak detected code, you may be able to resolve the problem without much effort. The easiest solution may be to remove and reaffix the gas cap. Once you do, clear the code on the OBD-II diagnostic scanner and drive for a few days.How many times should you click your gas cap? ›
Many manufacturers recommend tightening the fuel cap until it clicks three times, which is just a random number to make sure the cap is tight.Where is the gas cap sensor located? ›
The fuel tank pressure sensor is part of the fuel pump assembly and is mounted on top of the tank or inside the tank. It's part of the evaporative emissions system (commonly referred to as “EVAP”) and reads pressure in the fuel system to detect evaporative leaks, such as a loose or faulty gas cap.Will check engine light reset itself after replacing gas cap? ›
If you properly secure the cap, the system will then reset itself. It might take a couple of cycles but the check engine light will eventually go out.What is the first thing to check when the check engine light comes on? ›
Check your gas cap first. Many vehicles have a loose gas cap indicator that will be triggered before your check engine light comes on. If your gas cap is loose or the seal is not tight, the vapor leakage can cause your fuel system to trigger the check engine light.How worried should I be about check engine light? ›
Whether solid or blinking, when your “check engine” light has come on, it's best to take it to the repair shop. The light could indicate any number of things, most of which will require a mechanic to diagnose and address. Your car's performance is computer-monitored more than you might think.How long can you drive with a solid check engine light? ›
When the check engine light is solid, you can typically drive the car for hundreds of miles without an issue. Of course, that depends on which code is stored in the vehicle's computer. If an engine sensor is faulty, the car will usually use made up sensor values to keep running.Is ethanol gas better for your engine? ›
Even if mileage efficiency is reduced by 3%, ethanol-free gas is better for the engine. There have been reports of ethanol mixed gas leading to rusting engine parts. Ethanol, by nature, attracts water and can cause damage. There are gas stations across the U.S. that offer consumers a choice of ethanol vs.
Does E85 ruin o2 sensors? ›
Alcohol in the form of methanol and ethanol are oxygenates that are added to gasoline. E85 fuel is a mixture of 85% ethanol and 15% gasoline. E85 will burn cleaner and produce less degradation of the oxygen sensor.How long can a car sit with ethanol gas? ›
Ethanol-Blended Gas: Up to 3 Months
This type of gas has a shorter shelf life than pure gasoline and typically only lasts for up to three months. Unlike pure gasoline, ethanol-based gas easily absorbs moisture, which can lead to contamination.
Ethanol is more soluble in water than it is in gasoline. Therefore, if you add water to the gasoline and vigorously shake it, the ethanol will attach itself to the water. After it sits for a while, the gasoline and water/ethanol will make 2 distinct layers, and you can drain off the ethanol/water in a variety of ways.How do I protect my car from ethanol damage? ›
- Use ethanol-resistant hoses or nylon tubing to replace any plastic or rubber fuel lines.
- Replace fiberglass fuel tanks with a stainless steel tank.
- Use a water separator filter in the fuel line leading to the carburetor.
Ethanol is an excellent cleaner. It clears the engine, fuel lines, and fuel injectors of deposits. It's common for deposits to build up in the combustion chamber, fuel lines, fuel injectors, and a few other places within the engine.Does ethanol damage older engines? ›
But on the flip side, if you're driving an older vehicle, the ethanol may be causing problems. Alcohol is a highly corrosive substance that's hard on plastic, rubber, and even some metals, and over time it will eat away at your vehicle's most vulnerable components.Can E85 destroy my engine? ›
Ethanol has a corrosive action on fuel-system components, magnesium, aluminium and rubber. Running E85 on older model engines without tuning and replacing some components will ruin the engine in short time. Replacing fuel hoses, fuel pumps, gaskets, seals, fuel filters, fuel injectors, throttle bodies, etc.What will damage a O2 sensor? ›
Why do O2 sensors fail? Since the oxygen sensor is in the exhaust stream, it can become contaminated. Common sources of contamination include an excessively rich fuel mixture condition or oil blow-by in an older engine and engine coolant being burnt in the combustion chamber as a result of an engine gasket leak.Does ethanol give worse gas mileage? ›
Ethanol contains about one-third less energy than gasoline. So, vehicles will typically go 3% to 4% fewer miles per gallon on E10 and 4% to 5% fewer on E15 than on 100% gasoline.Is it better to keep gas tank full? ›
Keeping your gas tank more than half full limits the danger of running out of gas, and it can save you from costly repairs.
How much ethanol is too much for your car? ›
Both of those manuals said 15 percent ethanol is the maximum percentage you should be putting into those cars,” says Alex Knizek, auto engineer at Consumer Reports. “And another thing to remember is that ethanol isn't as energy dense as regular gasoline so you will see worse fuel economy with E15 gas.”