Nothing quite triggers the same feeling of dread as your car’s check engine light sparking to life. A lot of fear and uncertainty seems to orbit around this apparently innocuous little light, partly because no one seems to be sure what it means exactly.

We’ve decided it’s time to end the check engine light’s reign of terror. In our handy guide, we explain how these lights work, and explore some common reasons that they might switch on. Finally, we offer some practical advice about what you should do if this menacing little light makes its presence known on your dashboard.

What is a Check Engine Light and Why do We Need Them?

A car’s check engine light is essentially a diagnostic tool – it lights up when a car’s on board computer issues an error warning.

They come in many shapes and sizes, depending on the make, model, and age of your vehicle. Usually, the light is shaped in a pictogram of the engine, in amber or red, but can also be text reading ‘check engine’ or ‘service engine soon’. Check engine lights typically have two distinct stages: steady, indicating a minor fault, and flashing, indicating a severe fault.

Modern cars’ self-diagnostic abilities are impressive, and they came about largely in response to a piece of 1991 legislation. In that year, the California Air Resources Board required that all newly manufactured cars were equipped with an onboard emissions-monitoring system. Five years later, this monitoring system, called OBD-II (an acronym for ‘On Board Diagnostics’) was implemented throughout the US. Each system was fitted with a universal connector and communication protocols, so issues could be diagnosed by any garage with the right equipment. With OBD-II systems, the port is always located in the driver’s footwell.

OBD-II monitors many different aspects of a car’s operation, the main areas being:

  • Fuel injection
  • Ignition system
  • Automatic transmissions

Individual manufacturers may collect data on a whole host of additional components, but these are the basics. This diagnostic system eliminated the need for an annual ‘tune up’ with the mechanic, since on board computers constantly adjust the combustion process by gathering data from a number of sensors throughout the vehicle, and using it to combine the correct amount of gasoline with the correct ignition timing, in order to provide the most efficient burn possible. When problems are detected by OBD-II, this triggers the illumination of your car’s check engine light.

check engine

How do Check Engine Lights Work?

As well as allowing fuel to be burned more cleanly and efficiently, the OBD-II system created a new way for car problems to be diagnosed. When an OBD-II tool is connected to a car’s port, the ‘fault codes’ communicated can be interpreted by a mechanic. There are more than 3,000 generic fault codes, and they can indicate problems with anything from braking, to steering, to ignition, to suspension. Some cars even monitor their own tire pressure, and if you fancy trying this technology out for yourself, you can even purchase your own pressure monitoring kit to be retrofitted to your vehicle.

The ‘fault codes’ that mechanics can use to diagnose problems in your vehicle also trigger the illumination of the check engine light. When a sensor detects a problem, this data is relayed to the car’s on-board computer; the somewhat unimaginatively named Engine Control Unit (ECU). At first, the ECU interprets this fault code as ‘pending’. This means it could be a fluke, which resolves itself. Only if the ECU receives the same fault code on the next journey is a signal sent to the dashboard causing the check engine light to be illuminated.

The light remains illuminated as long as the fault remains. If it goes away on its own, the light will only switch off after three fault-free trips. Computer savvy ‘car hackers’ sometimes use OBD-II tools to clear fault codes from the ECU’s memory when the check engine light annoys them, but it’s best to avoid this strategy and find out what fault has been detected.

Common Reasons Your Check Engine Light is on

Now that we know a little more about how check engine lights work, it’s time to run through some of the most common faults that trigger its illumination. If your car’s check engine light has switched on, it doesn’t necessarily mean one of these issues is the culprit, but additional symptoms could help you understand what’s going on – you might even be able to perform minor fixes yourself.

  • Loose Gas Cap

You read that correctly – one of the most common reasons that check engine lights switch on is because of a loose gas cap. When you’re stopping for gas in a hurry, you might not screw the cap back into place properly, or even pull away with the cap still sitting on the car’s roof! Jokes aside, this apparently trivial issue can actually be quite damaging to a vehicle. Loose or missing caps allow fuel vapor to escape from the gas tank, and prevent the fuel inside from being kept a constant pressure. This leads to wastage, a less efficient burn, and more emissions. When you first notice the check engine light illuminate, pull over and check your gas cap is properly in place – this is especially likely to be the problem if the light comes on right after you fill up. If the gas cap is loose due to damage, or you’ve managed to misplace it altogether, a universal replacement can be had for only $15 from your local auto parts store, or online.

  • Failed Oxygen Sensor

Another common trigger for the check engine light is a malfunctioning oxygen sensor. These sensors help your car burn fuel cleanly and efficiently, by monitoring the amount of unburned oxygen in its exhaust system. With this information, the optimum mixture of air and fuel can be fed into the engine to improve efficiency. A failed oxygen sensor isn’t vital for your car to run, but you might notice that it’s guzzling more gas than usual. The sensors also help to reduce incomplete combustion, which produces of soot and poisonous carbon monoxide. A replacement oxygen sensor can be had for around $175, and labor costs can vary.

  • Ailing Catalytic Converter

The catalytic converter is another common culprit when check engine lights come on. It’s an exhaust system component responsible for transforming the harmful substances sometimes produced during combustion into safe, inert ones:

  • Poisonous carbon monoxide is converted into inert carbon dioxide
  • Nitrogen oxides, greenhouse gasses which can be harmful to humans and cause smog, are converted into harmless nitrogen and oxygen

These two reactions work in tandem with one another. If a car’s catalytic converter no longer works, chances are the vehicle will exceed emission caps for these chemicals. Replacing a catalytic converter can be a pricey affair, ranging between $200 and $600. You can help increase your converter’s lifespan by performing regular car maintenance – this ensures it isn’t dealing with more pollutants than usual, or exposed to dangerous temperatures.

  • Spark Plug or Ignition Coil Problems

Spark plugs and ignition coils are another potential culprit in our line-up. Ignition coils generate electricity, which the spark plugs then use to ignite fuel in a gas engine’s cylinders. If these components aren’t working as they should, fuel isn’t being burned as it should, and you can expect problems getting the car started, backfiring, stalling, poor fuel economy, and excessive engine noise. Luckily, new spark plugs only cost around $15, and having them installed is a simple affair. If you’re fairly DIY savvy, you can even replace a spark plug yourself at home.

Bear in mind that this won’t be an issue if you drive a diesel car, since diesel engines rely entirely on compression to ignite their fuel – they don’t have ignition coils or spark plugs.

  • Faulty Spark Plug Wires

Spark plugs themselves aren’t the only ignition system component that can cause the check engine light to switch on – problems with the wiring can also have the same effect. These wires conduct electricity from the ignition coils to the spark plugs, allowing them to ignite the fuel in the engine cylinders. Faulty wires don’t conducts electricity as well, meaning the spark plugs will sometimes fail to ignite the fuel. Symptoms of faulty spark plug wires include:

  • A drop in engine performance and power
  • Reduced fuel economy
  • Louder idling

Replacement wires can be had for $50, so this car issue shouldn’t break the bank.

  • Failure of the Mass Airflow Sensor

Another component that can illuminate the check engine light when it’s faulty is the mass airflow sensor (MAS). As its name suggests, the MAS monitors the amount of air entering the engine. Like the oxygen sensor, part of this component’s job is to help ensure fuel is burned as cleanly and efficiently as possible, by supplying the optimum air to fuel ratio to the engine. Without the MAS, a car will struggle at high altitudes where larger volumes of air are required to burn fuel due to lower oxygen density. If your MAS is ailing, you can expect to have trouble starting the car, reduced fuel economy, stalling, and excessive noise while idling. Purchasing a new MAF will set you back between $120 and $150, plus labor.

Engine trouble light

  • Newly Installed Alarms

If you have had an aftermarket alarm installed in your car, this can cause the check engine light to illuminate if it isn’t working properly. It’s fairly easy to tell if an aftermarket alarm is not performing as it should – it can:

  • Drain the battery
  • Prevent the vehicle from starting
  • Activate from only a slight disturbance, such as falling leaves

If you suspect that a new alarm could be behind your check engine light coming on, it will probably need to be reinstalled.

  • Vacuum Leak

Leaks in a vehicle’s vacuum system can also trigger the check engine light. Car engines naturally produce a vacuum as they operate, and this vacuum can be used to power brake boosters, windshield wipers, and several other accessories. It also helps direct harmful unburned fuel back into the engine, reducing emissions. This technology is particularly prevalent in cars from the 1980s and early 1990s – before electrical systems were capable of handling as many accessories as they are today. As you might expect, leaks are the most common vacuum system issue. The hoses that connect the intake manifold to various other components can become cracked over time, and sometimes need to be replaced. If your car has a vacuum leak you can expect an unusually high or fluctuating rpm when idling. New vacuum hoses cost only a few bucks each, but it could take a mechanic some time to track down the source of the leak.

  • Exhaust Gas Recirculation Valve Failure

Virtually every modern car has an exhaust gas recirculation (EGR) system, controlled by a valve. When the valve becomes clogged or otherwise fails, the check engine light will illuminate. We discussed the symptoms associated with this in an earlier blog entry. EGR valves can very often be removed, cleaned, and reinstalled to fix the problem – you can do this yourself or enlist the help of a mechanic. For a new valve, you can expect to pay around $125. It’s important to have a functioning EGR valve to ensure your car meets emission caps and runs as efficiently as possible.

In an EGR system, exhaust gasses are directed back into the engine, rather than sent straight to the exhaust pipes. This has a couple of benefits:

  • Any unburned fuel in the exhaust gases is sent back into the engine for combustion. This cuts down on the amount of gas being released into the atmosphere, and improves the engine’s fuel efficiency.
  • It reduces nitrogen oxide (NO) and nitrogen dioxide (NO2) emissions. These harmful pollutants form spontaneously from the nitrogen and oxygen in the air when temperatures are hot enough – as they are inside an internal combustion engine. Channeling these gases back into the engine’s cylinders reduces the proportion of oxygen in the cylinder, making for a smaller volume of combustible gas. This reduces the amount of heat released as the engine operates, cutting back on the formation of nitrogen oxides.
  • Flat Battery

A car’s battery has a lot riding on it – if the battery is drained enough, the car won’t even make it out of the parking lot! Faulty or flat batteries are another prime suspect when it comes to illuminating the check engine light. Symptoms of a faulty battery include:

  • A slower than usual crank when starting the car
  • Temperamental electronic equipment, such as power windows or windshield wipers
  • A separate dashboard light in the shape of a battery illuminates
  • A slight rotten egg smell emanating from the hood
  • A distended battery case

If the battery proves faulty, chances are you’ll have to purchase a new one, at a cost of around $100. If it’s simply out of juice, recharging it yourself is fairly straightforward. You can purchase a car battery recharger for as little as $15, which draws power from any outlet and pumps it into the battery. When recharging, be sure to:

  • Disconnect the battery before you begin, to minimise the risk of shocks
  • Disconnect the negative terminal first, and reconnect it last
  • Don’t charge the battery if the terminals appear corroded or otherwise damaged

Don’t Ignore the Light

If your car doesn’t feel or sound any different than normal when the check engine light comes on, there’s no need to instantly stop what you’re doing and make a bee-line for the nearest garage, but we urge you not to ignore the light either.

As we’ve seen, check engine lights are sometimes illuminated in response to fairly minor problems – a missing fuel cap or dodgy spark plug. Even if this is the case, ignoring the light is a bad idea, because now if a more serious problem arises, you no longer have the check engine light to warn you that something is wrong because it’s busy complaining about something else.

Proper car maintenance is much like health maintenance – if you ignore minor problems, they can quickly become much worse. The check engine light is no exception to this rule, and if it remains illuminated for a few trips, have your car checked out even if you haven’t noticed any other symptoms. This approach will save you considerable time and stress in the long run!

Malfunction or check engine car symbol

What to do When Your Check Engine Light Comes on

You may eventually have to seek the advice of a trusted mechanic when your check engine light rears its ugly head. In the meantime, however, there are troubleshooting steps you can take yourself to get to the bottom of your check engine light enigma:

  • Check the Gas Tank Cap

As mentioned earlier, a faulty, missing, or even incorrectly placed gas cap can illuminate the check engine light like christmas. If the cap appears to be in place, it’s often worth purchasing a new one to try. If this deactivates the light, great; if not, you’ve got a spare cap.

  • Check the Fault Code

You have a few different options for accessing this information: take your car to a mechanic, stop by an auto parts store, or do it yourself. Each approach has its own pros and cons:

  • Visiting your Mechanic – You’ll need to pay a diagnostic fee to use this service in your average shop, but a local mechanic is likely to waive the fee if you agree to have the necessary work done at that particular garage. This one-stop-shop approach can be the most convenient option, and is probably best if you notice that your car feels or sounds different. This option is also your best bet if the check engine light is flashing on and off, as this denotes a major, rather than minor, problem.
  • Visiting an Auto Parts Store – Most auto parts stores offer free diagnostics in the hope that customers will buy any necessary components right there and then. This option could save you money compared to the mechanic, as you may find you can fix whatever the problem is yourself. Before your visit, be sure to check the store can accommodate the make and model of your car – some have unique onboard computers.
  • Checking Yourself – Alternatively, you could invest in an inexpensive OBD-II scanner and check out the fault code for yourself. If you have an older car that frequently exhibits minor problems, this is a very sound investment. It can also be useful for figuring out the extent of the problem when you’re on the go, or far away from your regular mechanic or auto shop.

Once the problem has been identified, it’s a simple case of fixing the problem if you can, or seeking the help of a trusted mechanic if it’s beyond your personal knowledge.

Using an OBD-II Scanner

Checking which fault code has caused your check engine light to illuminate is a relatively simple procedure:

  • Acquire the tools – Purchase an OBD-II scanner and ensure it’s compatible with your car. Some scanners can transmit information to your mobile device, making them more accessible to use. This type of system will set you back around $15.
  • Locate the Connector – Your vehicle handbook should help with this. Look out for a 16-pin D shaped connector, usually located close to the steering wheel, and often in the driver’s footwell.
  • Turn on the Ignition – Don’t start the engine, but turn the key until the ignition switches on.
  • Launch the Diagnostic Software – Open up the program or application that came with the scanner, on your laptop, tablet, or smartphone, and follow the instructions it provides.
  • Interpret the Codes – Most software will provide a numerical code, along with an explanation about what it means.

Using these scanners can help you understand the problem – you might even be able to avoid a costly trip to the mechanic.


  1. Check Engine Light: What You Need to Know,
  2. What does the check engine light usually mean? How Stuff Works
  3. What Does the Check Engine Light Really Mean? Consumer Reports
  4. Don’t Ignore Your Check Engine Light: How to Handle It, Nerd Wallet