Check Engine Light (Complete Guide)
Updated: Jan 14
The check engine light (CEL) is a dashboard warning light that is found on most modern cars. It is typically labelled with the words "check engine" or a picture of an engine, and it is designed to alert the driver to potential problems or issues with the car's engine or emissions control system.
The check engine light is also known by several other names, including the following.
Engine light: This is a common name and it's essentially a shortened version of the check engine light.
Malfunction indicator light (MIL): This is a technical term for the check engine light, as it indicates that the car's computer has detected a malfunction or problem with one of the car's systems.
Service engine soon light: This is another common name for the check engine light, as it indicates that the car needs to be serviced or repaired.
Engine management light: This is another technical term for the check engine light, as it indicates that the car's engine management system has detected a problem or fault.
Idiot light: This is an old-fashioned name for the check engine light, as it was originally designed to alert "idiot" drivers to potential problems with their cars.
Regardless of what it is called, the check engine light serves the same purpose; to alert the driver to potential problems or issues with the car's engine or emissions control system.
Table of Contents:
Why Do Vehicles Have a Check Engine Light?
When the check engine light comes on, it typically indicates that the car's computer has detected a problem or fault with the engine or emissions control system.
This problem could be caused by various issues, it could be something minor such as a loose or faulty fuel cap, a faulty sensor, or it could be a more severe problem with the engine or emissions control system.
If the check engine light comes on, it is essential to take notice and take appropriate action.
Depending on the severity of the problem, this may involve stopping the car and turning the engine off as soon as it's safe and checking for any visible issues.
If the problem is not immediately apparent and there are no drivability issues, the driver should consult the car's owner's manual and have the issue investigated.
Ignoring the check engine light can further damage the car and put the driver and passengers at risk.
In some cases, driving a vehicle with the check engine light illuminated may be illegal, depending on the type of the problem and the laws in your country. For example, it may be illegal if the vehicle has a problem with the catalytic converter and is producing excessive emissions.
Therefore, it is essential to have the car checked and repaired as soon as possible if the check engine light comes on.
Types of Check Engine Light
The check engine light on your might be intermittently illuminated, continuously illuminated, or blinking / flashing; each version of the CEL has its own purpose.
Intermittent: If your check engine light comes on from time to time, take attention to any changes in your vehicle's performance. If you notice a difference, attempt to drive it as little as possible until it can be fixed. Most likely, something is about to fail, and you risk your vehicle not starting again; if the problem is intermittent, it's usually caused by a sensor failing or an electrical problem.
Constant illumination: If the check engine light is constantly illuminated and not blinking, your vehicle's inbuilt diagnostic system has discovered a significant enough issue to illuminate the check engine light. Even if your vehicle seems to be in good working order, it is best to have it inspected as soon as possible to prevent further issues. If possible, avoid driving or starting the car.
Blinking / flashing: If your check engine light is flashing or blinking, it typically implies your vehicle has a serious problem and you should pull over as soon as possible. It is typically preferable to have the car towed or recovered and repaired. You should not drive or start the car.
What to Do if Your Check Engine Light Comes On
When the check engine light comes on, it is important for drivers to take notice and address the issue as soon as possible. Ignoring the check engine light can lead to further damage to the vehicle and potentially more costly repairs down the road.
Below are some tips for what drivers should do when the check engine light comes on.
Don't panic: While the check engine light can be alarming, it doesn't necessarily mean that there is a major problem with your vehicle. It could be a simple issue such as a loose fuel cap or a faulty sensor. Drive safely and pull over somewhere where it is safe to do so.
Check for other warning signs: If you notice any other warning signs such as strange noises or decreased performance, it could be a sign of a more serious issue. If you notice any serious issues or another warning light (such as the red ABS light) you should pull over and stop the engine when it's safe to do so.
Decide whether you can drive the vehicle or if it needs to be recovered: If there are serious issues, your vehicle has leaking fluid, it has other warning lights or the check engine light is flashing you should not drive the vehicle. If there are no obvious issues, no leaking fluids, and no other warning lights you may be fine to drive the vehicle.
Get the issue checked out: As soon as you notice the check engine light, it is important to get the issue checked out by a mechanic.
Use your own judgement to decide if the vehicle is safe to continue driving or if you need the vehicle recovered or towed.
Potential Consequences of Ignoring the Check Engine Light
Ignoring the check engine light can have serious consequences for your vehicle. Listed below are a few potential consequences of ignoring the check engine light.
Reduced performance: If the check engine light comes on, it's likely that there is an issue with your vehicle's engine or powertrain components that is affecting its performance. Ignoring this issue can lead to reduced performance, such as a loss of power or poor fuel economy. The ECU may have also triggered your vehicle to enter limp mode.
Increased emissions: Many of the issues that trigger the check engine light are related to the emission control systems of a vehicle. Ignoring these issues can lead to increased emissions, which can have negative environmental consequences and may also be illegal in some areas.
Potential engine damage: If the check engine light is ignored for an extended period of time, it's possible that the issue could lead to more serious problems with the engine or powertrain. This could result in costly repairs or even the need to replace the engine altogether.
By addressing the check engine light as soon as it comes on, you can help to ensure that your vehicle is running at its best and avoid these potential consequences.
It's also a good idea to regularly maintain your vehicle and address any issues with the engine or exhaust system as soon as they arise to help prevent the check engine light from coming on in the first place.
Failsafe Mode (Limp Mode)
If you notice a consistent drop in performance (reduced acceleration, reduced top speed, limited RPM range, etc.) when the check engine light is illuminated, your vehicle may have entered failsafe mode (also called "limp mode") to avoid further damage.
In the case of failsafe mode being triggered, you should avoid starting or driving your vehicle until the issue is resolved.
The Role of the OBD System
The OBD (on-board diagnostics) system is a computer-based system that monitors the performance of a vehicle's engine, transmission, and other components.
It uses sensors and other electronic devices to gather data about the vehicle's performance and sends that information to the vehicle's on-board computer.
One important function of the OBD system is to monitor the various systems and components of the vehicle for any issues or malfunctions. If a problem is detected, the OBD system will trigger the check engine light to alert the driver of the issue.
The check engine light is a warning indicator that is typically located on the dashboard of a vehicle and is designed to alert the driver of a potential problem with the vehicle.
The check engine light can be triggered by a variety of issues, ranging from a loose fuel filler cap to a more serious problem such as a faulty sensor or engine damage.
When the check engine light comes on, it is important for the driver to take notice and address the issue as soon as possible. Ignoring the check engine light can lead to further damage to the vehicle and potentially more costly repairs down the road.
To determine the cause of the check engine light, an OBD scan tool is used to retrieve diagnostic trouble codes (DTCs) from the OBD system. These codes can provide information about the specific issue that triggered the check engine light.
History
The history of the check engine light can be traced back to the early days of the automotive industry, when cars were first equipped with internal combustion engines.
Initially, cars did not have any kind of dashboard warning lights or indicators, so drivers had to rely on their own observations and knowledge of the car's systems to diagnose and fix problems.
As cars became more complex and sophisticated, the need for a way to alert drivers to potential problems became increasingly important.
In the 1950s, car manufacturers began to experiment with dashboard warning lights, and the first check engine light was introduced by the American Motors Corporation (AMC) in the late 1950s.
This light was called the "idiot light" and was designed to alert drivers to any issues with the car's engine or emissions control system.
Over the years, the check engine light has become a standard feature on most modern cars. It is typically located on the dashboard, along with other warning lights and indicators, and is labeled with the words "check engine" or a picture of an engine.
The check engine light is triggered by the car's computer when it detects a problem or fault with the engine or emissions control system. It is usually accompanied by the ECU generating OBD fault codes in response to an issue, these can be viewed using an OBD scanner.
Today, the check engine light is an essential part of a car's safety and diagnostic system. It helps to alert drivers to potential problems with their cars and enables them to take appropriate action to fix the problem and avoid further damage to the car.
Triggering of the Check Engine Light
There are various reasons for triggering the check engine light. Those reasons that are not caused by an actual electrical or mechanical issue are described below.
Odometer Triggering
Some cars built between the late 1990s and the mid-2000s include a MIL that illuminates based on the odometer reading regardless of engine performance.
In many Mazda models, for example, the light will illuminate at 80,000 miles and stay illuminated without issuing a computer issue code.
Volvo has a light called "lambda", which is another term for an oxygen sensor. This was illuminated to notify the driver to replace the oxygen sensor.
Some Chrysler cars produced between 1973 and 1976 have a similar odometer-triggered reminder "Check EGR", which was reset following service at a Chrysler dealership.
These are just some examples of mileage based triggering of the check engine light.
False Triggering
Higher concentrations of methanol/ethanol (or other additives) than the engine can tolerate may set off the MIL. These burn differently than gasoline, and the EFI system may incorrectly interpret the oxygen sensor's values.
In early EFI systems, oxygenated gasoline may cause erroneous triggering.
Testing of the Warning Lights
The MIL is also lighted prior to starting the engine, along with other dashboard warning indicators, to show that the warning light is operational and not burnt out.
If there are no detected defects, the light will switch off when the vehicle is started.
Cranking of the Engine
If the engine is cranked but fails to start after restoring the ignition to "on", the check engine light will illuminate. This is mainly due to not allowing the engine adequate cranking time to start in most new vehicles.
Before fuel injection, multiple cranking efforts were necessary to start the vehicle. If the engine fails to start on the second or third try, inspection of the engine may be necessary.
Stalling
If the engine stalls unexpectedly or is caused by an overload, such as in a manual transmission vehicle, the check engine light will glow until the engine is restarted.