How To Reset a Check Engine Light - Why Your Check Engine Light Is On?

Written By: Don Dodi
Fact Checked By: Kristen Brown
Reviewed By: Diego Rosenberg

Basically if there’s a tiny problem with the engine then the check engine light warns you by this light.

And it is actually the part of the On-Board Diagnostics System (OBD-II), which actually became compalsory in all vehicles sold in the U.S. from 1996 onward and the it actually constantly monitors your engine and emissions systems.

So, whenever it detects a problem, even something as small as a loose gas cap or a failing oxygen sensor, then it triggers the light.

As well as, according to the CarMD’s 2023 vehicle Health Index, nearly 65% of the check engine alerts are due to the minor or moderate issues that drivers can easily diagnose themselves using an OBD-II scanner, which is available for less than $30.

Yes, it is possible to reset the check engine light on your own, but when you do this without fixing the problem then it can lead to the worsened performance, failed emissions tests or costly repairs later on as well.

So, as per EPA regulations, a functioning check engine light is actually legal requirement: and it must illuminate when the emissions-control malfunctions that boost the pollution over 150% of certified limits as well.[¹]

Now, this is the guide specifically focus on How To Reset a Check Engine Light and here’s you’ll know safe, 100% legal and effective ways to reset it only after resolving the actual issue.

Understanding the Check Engine Light

1. What the Check Engine Light Indicates

The check engine light (CEL) is a warning signal from your car’s onboard computer system, also known as OBD-II (on-board diagnostics version 2).

When something goes wrong with the engine or emissions system, the computer turns on this light to alert you that your car needs attention.

This light is your vehicle’s way of saying, “Something isn’t right, please check.” Although the light itself doesn’t tell what’s wrong, it does indicate that there is a diagnostic trouble code (DTC) stored in the system, which you can read using a scan tool.

Ignoring this warning could result in reduced fuel economy, increased emissions, or serious engine damage over time.

2. Emissions System

A large number of check engine light warnings are tied to the emissions system, which is responsible for reducing the pollution your car releases into the air.

Components such as the catalytic converter, evaporative emissions (EVAP) system, oxygen sensors and EGR valve work together to clean exhaust gases. If any of these components begin to fail or a leak is detected, the car’s computer activates the CEL.

For example, the U.S. Environmental Protection Agency (EPA) emphasizes that a malfunction in the emissions system can lead to increased air pollution and failure of emissions tests, which in some states may make it illegal to drive your car until it is repaired.

3. Engine Performance

The Check Engine Light can also indicate problems affecting your engine’s operation, such as an engine misfire, faulty spark plugs, or fuel mixture issues.

These problems don’t always produce immediate symptoms, but over time, they can cause rough idle, loss of power, or poor fuel economy.

According to AAA, these engine-related problems account for more than 25% of all check engine light alerts. Identifying and correcting them early can prevent costly repairs and maintain smooth performance.

4. Sensors and Onboard Diagnostics (OBD-II)

Modern vehicles rely on a network of sensors to monitor everything from air-fuel ratio to exhaust emissions. Major sensors include oxygen sensor (O2 sensor), mass airflow sensor (MAF), throttle position sensor (TPS), etc.

When these sensors detect a problem or begin to report incorrect data, the OBD-II system logs a code and turns on the light. Even minor problems—like a weak battery or a temporary malfunction in a sensor—can cause the CEL to display.

Thankfully, OBD-II scanners are widely available and easy to use, helping you identify the exact trouble code stored in your car’s memory.

5. Common Reasons the Check Engine Light Turns On

According to data from CarMD and RepairPal, here are some of the most common causes of a check engine light:

Loose or damaged gas cap (responsible for up to 10% of CEL triggers)
Faulty oxygen sensor (can reduce fuel economy by 20-40%)
bad catalytic converter
Faulty spark plug or ignition coil
Mass airflow (MAF) sensor failure
Leaks or errors in the EVAP system
Problems with the battery or charging system

The severity of these problems can vary – from minor and easy to fix, to more complex and requiring professional help.

6. Solid vs. Flashing Check Engine Light

Understanding the behavior of light is just as important as paying attention to it:

A solid check engine light usually indicates a non-urgent problem that still needs attention, such as a loose gas cap or faulty sensor.
On the other hand, a flashing check engine light indicates a more serious problem, often due to active misfiring in the engine and damage to the catalytic converter. In such a case, it’s important to stop driving and have the car checked immediately.

According to the National Institute for Automotive Service Excellence (ASE), a flashing CEL is a red warning that continued driving could cause serious engine damage.

Diagnosing the Cause First

1. Importance of Using an OBD-II Scanner

Before attempting to reset the check engine light, the most important step is to diagnose the root cause using an OBD-II scanner (On-Board Diagnostics, version 2).

Every car built after 1996 has this system, which constantly monitors your engine, transmission and emissions systems. When something goes wrong, it stores a trouble code in its memory.

An OBD-II scanner reads these codes and gives you clues to the exact problem. Using one is not only smart, but necessary—because resetting a light without understanding what caused it to come on can hide serious problems and cause even more damage.

These scanners are affordable (less than $25 online) and easy to use, even for beginners. Simply plug it into the OBD port under the dashboard and it will show a code like P0301 (cylinder 1 misfire) or P0455 (EVAP system leak). This code is your vehicle’s way of communicating with you directly.

2. Where to Get a Free Scan (AutoZone, Local Auto Shops & Us)

If you don’t have a scanner, don’t worry — you don’t need to buy a scanner right away. Many auto parts stores, such as AutoZone, O’Reilly Auto Parts, Advance Auto Parts, NAPA and to us as well, offer free check engine light diagnostic scans.

Simply drive in and an employee will plug in a scanner, read the code and give you a printout with possible causes and solutions.

Some local mechanics and oil change shops also offer free or reduced-price diagnostic services as part of promotional services. Using this free service can help you save money and get clarity without any estimates.

3. Interpreting Diagnostic Trouble Codes (DTCs)

The codes obtained from an OBD-II scanner are called diagnostic trouble codes (DTC) and each code begins with a letter (P, B, C, or U) followed by four digits. For example:

P0420 – Catalyst System Efficiency Below Threshold
P0171 – System too weak (Bank 1)
P0300 – Random/Multiple Cylinder Misfire Detected

These can be broken down as follows:

The letter indicated this system:
- P = Powertrain
- B = Body
- C = Chassis
- U = Network (communication systems)
The first digit tells you whether the code is generic (0) or manufacturer-specific (1).

Understanding these codes is the first step toward making an informed decision before attempting repairs yourself or visiting a mechanic.

Many OBD apps or websites (such as OBD-Codes.com or manufacturer service guides) can help you understand what each code means.

4. Why You Should NOT Clear the Light Before Fixing the Issue

It might be tempting to just clear the warning and move on, but clearing the check engine light without fixing the problem is a bad idea.

First, it doesn’t solve the root problem—it just temporarily hides it. This means the problem can get worse without any obvious warning.

Second, many states mandate emissions testing as part of their vehicle inspection process. If the OBD system shows a recent code clearing or incomplete readiness monitor, your car may automatically fail the inspection, even if the light is off.

Third, if the problem returns—and it often does—the light will come back on, possibly with additional or more serious codes.

According to the EPA and the National Highway Traffic Safety Administration (NHTSA), improper maintenance or ignoring warning systems like the CEL can result in increased emissions, decreased fuel efficiency and long-term damage to the engine. You should always fix the cause, not just the symptoms.

How To Reset a Check Engine Light – After Fixing the Problem

After properly identifying and resolving the cause of the check engine light with the help of an OBD-II scan tool or professional diagnostics, the final step is to reset the light. Resetting does not fix the problem – it only removes the warning from the dashboard.

That’s why it’s important to do this only after repairs. There are four common and effective ways to reset a light, each with different uses and potential effects.

1. Using an OBD-II Scanner

This is the most accurate and safest way to reset the check engine light.

Plug into the OBD-II Port:
Locate the OBD-II diagnostic port, usually under the dashboard on the driver’s side, near the steering column.
Turn the Ignition On (Engine Off):
Switch the key to the “On” position (turn on the dashboard lights), but do not start the engine.
Read and Clear Codes:
Use the scanner to read the stored trouble codes. If the problem has already been fixed, you can select the “Clear Codes” or “Erase DTCs” option.
Confirm the Light is Off:
Turn off the ignition, then restart the car to make sure the check engine light stays off. If it turns on again, the problem may still exist.

When This Method Is Appropriate:

After completing a confirmed repair.
To verify that no extra codes are stored.
When Preparing for State Emissions Testing.
When you want a clean slate for future diagnostics.

OBD-II scanners are inexpensive and available at auto stores or online. Advanced models can even connect to your smartphone for added convenience.

2. Disconnecting the Battery

This is an old method that still works for many vehicles today. It clears the car’s memory, including stored codes.

Turn Off the Ignition:
Make sure the engine and all electronic devices are completely turned off.
Disconnect the Negative Terminal:
Loosen the nut on the negative terminal (black) of the battery using a wrench and remove the cable.
Wait 15–30 Minutes:
This gives the vehicle’s electronic systems enough time to completely reset.
Reconnect and Start the Engine:
Reconnect the negative cable, tighten it and start your vehicle to check if the light goes out.

Possible Side Effects:

Resets radio presets and dashboard clock.
Clears trip data and memory from the ECU.
Engine performance may be temporarily affected as the car re-learns driving patterns.

This method is simpler but less accurate than an OBD-II scanner.

3. Letting the Light Turn Off Automatically

If the problem that activated the Check Engine Light was minor and intermittent, the light may turn off automatically once the system confirms that the problem no longer exists.

Drive Cycle Explanation:

A drive cycle is a specific pattern of driving conditions that your car’s computer needs to check all sensors and systems. It usually includes:

Starting with a cold engine.
Being idle for a few minutes.
Driving at different speeds (e.g., city and highway).
Accelerate, decelerate and stop.

How Long It May Take:

It may take up to 50 to 100 miles under varying driving conditions for the system to complete a full drive cycle and automatically clear the light, provided the problem has been completely resolved.

When This Method Works Best:

After tightening the loose gas cap.
When dealing with temporary sensor errors.
Auto-fix minor errors that no longer meet warning threshold.

This method requires patience, but does not require any equipment or intervention.

4. Pulling the ECU Fuse

This is a more advanced method, usually used by experienced DIYers or mechanics.

What It Does:

Pulling the ECU (engine control unit) fuse interrupts the power supply to the vehicle’s computer, causing it to reset. This clears all stored fault codes and may turn off the check engine light.

How to Locate and Remove the ECU Fuse:

Refer to the Owner’s Manual:
Find the location of the fuse box and identify the ECU or ECM fuse (usually labeled).
Turn Off Ignition:
Always turn off the engine before removing any fuses.
Remove the ECU Fuse for 10–15 Minutes:
This will clear the system memory and require a reset.
Reinsert the Fuse and Restart the Car:
Check the dashboard to see if the CEL has gone off.

This method avoids completely disconnecting the battery, but also carries some risks, such as losing stored settings. Use this only if you are confident in the layout of the system and how the fuses work.

Read More:

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When You Shouldn’t Reset It Yourself

While resetting the check engine light may seem like a quick fix, there are situations where doing it yourself could be risky or even harmful to your vehicle.

In some cases, resetting the light without proper repairs can hide serious problems, delay needed repairs and cause more costly damage later.

Below are some specific situations in which you should not turn off the check engine light yourself – but instead seek professional help.

1. Flashing Light = Urgent Engine Damage Risk

If your check engine light is flashing, it’s a sign of a serious, active engine misfire — a condition in which one or more cylinders aren’t firing properly.

This can cause raw, unburned fuel to enter the exhaust system, which can damage the catalytic converter, one of the most expensive parts of your emissions system.

According to the National Institute for Automotive Service Excellence (ASE), continued driving with a flashing CEL can cause serious engine damage, increased emissions and significant repair costs.

In this case, you should stop driving immediately and take the vehicle to a mechanic for diagnosis.

2. Recurring Check Engine Light After Reset

If you reset the check engine light and it comes on again shortly after – often within a day or a few miles – that’s a clear sign that the original problem hasn’t been resolved.

The OBD-II system is designed to constantly monitor sensors and systems and will turn the light on again as soon as a malfunction is detected again.

Recurring codes like P0420 (catalytic efficiency below limit) or P0171 (system too lean) could indicate ongoing sensor issues, vacuum leaks or exhaust problems.

Re-erasing the code won’t solve the problem – in fact it may cause you to ignore damage that will only get worse over time.

3. Airbag/SRS or Transmission-Related Codes

The check engine light usually indicates powertrain and emissions-related problems, but in some cars, it could be accompanied by codes related to the transmission or even a warning light from an SRS (supplemental restraint system), like an airbag system.

These are critical safety systems that should never be overlooked or cleared without a thorough check. For example, some transmission problems can cause the car to run in “limp mode,” limiting power to avoid damage — and clearing the lights can disable this protection.

Likewise, clearing airbag warnings without repairs may leave you without functional crash protection in the event of a collision. In such cases, always consult a certified technician who has the correct diagnostic tools and expertise.

4. Importance of Professional Diagnostics for Complex Issues

Modern vehicles are equipped with hundreds of sensors and control modules, all connected through complex computer networks.

Some problems may seem minor at first glance, but they could be signs of larger system failures, such as faulty wiring, corrupted ECU software, or multi-system faults that require in-depth troubleshooting.

Professional mechanics have access to OEM diagnostic tools, technical service bulletins (TSBs) and live data monitoring to pinpoint the root cause. Performing a reset yourself may erase this valuable data and make future diagnostics difficult.

According to a report from AAA, nearly 70% of drivers who attempted to self-diagnose and repair check engine light problems ultimately had to visit a mechanic for further assistance.

In complex or recurring situations, the best option is to let trained professionals handle the inspection and repair process – specially if safety or driving ability is at stake.

Tips to Prevent Future Check Engine Lights

The check engine light can be annoying, but the good news is that many of the problems that trigger it can be prevented with a few simple habits and routine care.

By paying attention to a few key maintenance tips, you can reduce the likelihood of unexpected engine warnings, save money on repairs and keep your vehicle running smoothly for longer.

Here are some of the most effective, easy tips to help you avoid check engine light problems in the future.

1. Tighten the Gas Cap After Fueling

The most common and easily preventable cause of a Check Engine Light is a loose or damaged gas cap.

Your car’s EVAP (evaporative emissions control) system is designed to keep fuel vapors sealed in the tank.

If the gas cap isn’t properly tightened, it can trigger a code P0457 or P0455, which indicates a vapor leak. According to CarMD’s 2023 Vehicle Health Index, about 10% of check engine light cases are related to gas cap problems.

Make it a habit to turn the gas cap at least three times until you hear a click after each fill-up. If the cap is broken or worn, it’s easy to replace it with a new one (usually less than $15).

2. Regular Maintenance

Performing regular maintenance is one of the best ways to prevent the check engine light from coming on. This includes:

Change the oil and filter every 5,000-7,500 miles (or as recommended in your car’s manual).
Change air filters regularly to avoid air flow problems.
Inspecting spark plugs and ignition coils to prevent misfires (often associated with code P0300).
Checking fluid levels, including coolant and transmission fluid.

Neglecting regular maintenance can damage sensors, reduce fuel efficiency and even damage the engine, causing CEL problems. According to AAA, a lack of basic maintenance is responsible for more than 30% of vehicle breakdowns.

3. Use Proper Fuel and Oil

Using the wrong type of fuel or oil can cause engine knocking, poor combustion and worn parts, which can confuse sensors and cause the Check Engine Light to come on. Always follow your vehicle manufacturer’s recommendations:

Use the correct octane level for gasoline (87, 89, or 91+)
Choose the correct engine oil grade (e.g., 5W-30 or 0W-20) and use only API-certified brands.

Using low-quality or substandard products can lead to increased deposits, poor fuel combustion and damage to the catalytic converter, which is one of the most expensive parts to replace.

The Engine Oil Licensing and Certification System (EOLCS) makes sure the oil you’re using meets manufacturer standards — check for the “API Starburst” symbol on the bottle.

4. Replace Worn Sensors (O2, MAF) as Needed

Sensors such as oxygen sensor (O2 sensor) and mass airflow sensor (MAF sensor) play a vital role in managing the air-fuel mixture, emissions and engine efficiency.

Over time, these sensors can become dirty, wear out, or start sending incorrect data to the engine control unit (ECU). For example, a bad oxygen sensor can reduce fuel economy by up to 40% and is responsible for millions of CEL triggers (specially codes P0133 or P0420) each year.

Replacing these sensors typically costs $100-$300, but can restore their performance and help pass emissions testing. Cleaning the MAF sensor with a special spray every 15,000-30,000 miles can also prevent problems.

Conclusion – How To Reset a Check Engine Light

So, when you fix the cause behind the check engine light is similar such as: treating the illness instead of hiding the symptoms and that’s the way that you can avoid any costly repairs later on.

Although resetting it can be done with a scanner or some simple tools, the real key lies in understanding the underlying cause and fixing it.

Whether the gas cap is loose or the oxygen sensor is bad, modern vehicles rely heavily on electronic systems to monitor performance and emissions.

Repeatedly ignoring or clearing the light without fixing the problem can not only harm the engine and reduce fuel economy, but it could also cause your vehicle to fail an emissions test, specially in states with strict inspection laws.

According to industry statistics, about 50-60% of check engine light alerts are caused by minor problems that can be fixed quickly and affordably – but leaving them unchecked can lead to costly repairs.

Devices like OBD-II scanners, free code readings at auto parts stores and a little maintenance knowledge can save hundreds, even thousands of dollars over time.

Most importantly, never reset a flashing check engine light or lights associated with critical systems like the transmission or airbags without a proper diagnosis from a professional.

So, that’s all from this guide and now you can ask questions in the comment section below.

Frequently Asked Questions

Q1. How do I delete check engine light codes?

Answer: The best and most reliable way to remove a check engine light code is to use an OBD-II scanner. This small device plugs into a port under your dashboard and allows you to read codes stored in your car’s computer. Once you fix the problem that caused the warning, you can erase or clear the codes from memory using a scanner, which will turn off the check engine light. Some advanced scanners or mobile apps will even tell you the meaning of the code, so you can decide whether you can fix the problem yourself or seek help from a mechanic. Removing the code without actually fixing the issue is not recommended, as the problem will likely reoccur and cause bigger issues later.

Q2. How long does it take for a check engine light to reset?

Answer: If the problem that caused the Check Engine Light to come on has been resolved, the light automatically resets after the car has completed a certain period of driving, called a drive cycle. A complete drive cycle includes starting the engine from cold, driving at various speeds, accelerating, decelerating and letting the car idle. This allows the car’s computer to double-check all systems and sensors. On average, it may take a few days of driving for 50 to 100 miles for the light to turn off on its own to see if the problem is indeed gone. However, if the light stays on, the computer is still detecting a problem and it’s best to have the car scanned again.

Q3. Can you clear an engine code without a scanner?

Answer: Yes, it is possible to clear engine codes without a scanner, but it’s not always the best method. A common method is to disconnect the car’s battery, which resets the vehicle’s computer. To do this, you need to turn off the engine, disconnect the negative terminal of the battery, wait for about 15-30 minutes and then reconnect it. This may clear the engine codes and turn off the light. However, this method doesn’t work on all vehicles, specially newer ones and it may also reset important settings like your clock, radio and seat memory. More importantly, if the problem still persists, the light will come back on immediately after you drive again. Using a scanner is the safest and most accurate way to clear codes.

Q4. Will disconnecting the battery clear permanent codes?

Answer: No, disconnecting the battery will not clear permanent codes. Permanent codes are a special type of diagnostic trouble code (DTC) that your vehicle stores when a serious problem is detected – even if the problem is fixed. These codes are stored in your car’s memory until the OBD system confirms after several drive cycles that the problem has indeed been resolved and no longer exists. This is a safety feature built into modern vehicles that prevents tampering and ensures emissions problems are fully repaired before clearing the code. So even if you disconnect the battery, the permanent codes will remain until the car confirms that the system is working properly over time.

Q5. Is it illegal to remove a check engine light?

Answer: Yes, in many areas, it is illegal to remove, disable, or tamper with the check engine light – specially in countries like the United States, where vehicles have to meet strict emissions standards set by the EPA. The check engine light is part of your vehicle’s onboard diagnostics system and needs to remain functional so it can alert drivers to serious engine or emissions problems. If a mechanic or seller removes or conceals this warning light in order to sell a faulty car, it could be considered fraud or vehicle tampering, which is punishable by both fines and legal action. For your safety and the environment, the Check Engine Light should never be disconnected or simulated – always resolve the problem instead.

References:

[1] US EPA. Final Rule: Control of Air Pollution From Motor Vehicles and New Motor Vehicle Engines; Modification of Federal On‑Board Diagnostic Regulations. 1998.
https://www.epa.gov/regulations-emissions-vehicles-and-engines/final-rule-control-air-pollution-motor-vehicles-and-new

Don Dodi

Guys, I’m a car audio enthusiast and customization expert and I love clear sound and clean installation. Plus, I have spent years helping people build their dream audio setups. And whenever I’m working on cars, I probably listen to music and thinks about the next big upgrade.