Hey, Why Is My Car's Check Engine Light On?

The Check Engine light appears because of:

  • Engine misfire
  • Evaporative emission leak
  • Loose gas cap
  • Faulty airflow sensor
  • Faulty catalyst
  • Lean codes
  • Faulty variable valve timing (VVT)

If you are short on time and want to fast-track the process to professional help, click Yes

Four common causes for a “check engine” light on the vehicle and their related parts.
Does a "check engine" light on mean that you're losing fuel mileage? This single light has hundreds of reasons to glow. So, it's time to learn about the performance symptoms, like surging, hesitation, chugging, and low power it creates. There are plenty of common issues that activate the check engine light.

Is Your Car's Check Engine Light Constantly On?

The yellow check engine light commonly warns that the engine releases more emissions than expected, often due to a catalytic converter failure. This causes engine misfires, poor fuel mileage, low power, and more. But there are alternative common causes for this issue.

The most common causes for the Check Engine light are:

  • Engine misfire: When an engine misfires, it fails to ignite the air-fuel mixture properly, leading to a loss of power and rough running. The onboard diagnostics system detects this issue and triggers the check engine light.
  • Evaporative emission leak: An evaporative emission (EVAP) leak can cause the check engine light to illuminate because it disrupts the vehicle's emissions control system. A leak in the EVAP system, allows fuel vapor to escape, leading to increased emissions. 
  • Loose gas cap: If the gas cap is not properly tightened or is missing, it can allow fuel vapors to leak out, triggering the check engine light.
  • Faulty airflow sensor: The airflow sensor measures the amount of air entering the engine to help the system adjust the fuel injection for a perfect air-fuel mixture. If the sensor fails, it'll cause a bad mixture and trigger the check engine light.
  • Faulty catalyst: The catalytic converter converts harmful gases into less harmful emissions. If the catalyst is faulty, it increases emissions and triggers the check engine light.
  • Lean codes: Lean codes can cause the check engine light to illuminate because they indicate a problem with the air-fuel mixture in the engine. A lean condition occurs when there is too much air and not enough fuel in the air-fuel mixture, leading to incomplete combustion.
  • Faulty variable valve timing (VVT): The VVT system controls the timing of the opening and closing of the engine's valves. If the VVT system is faulty, it may not operate correctly, leading to issues such as rough idle, reduced power, and increased emissions, triggering the check engine light.

Stay alert if the check engine lights up on your car's dashboard. Your vehicle might be under progressive damage and can end up requiring hefty repairs in the future; it'll also show considerable fuel loss. So please, keep track of your car's odd behaviors and browse through our articles to find out the information you'll need to share with your go-to auto shop.

Let's Get To The Bottom Of It!

  • Q: Is your engine misfiring?

    A technician can clear the "check engine" light using specific tools. However, this would only mask the problem. It's essential to understand that any engine computer alert is troublesome and can develop into something worse, costing safety and money. In addition, the engine computer has a feature that stores information concerning the vehicle's overall condition when the problem first popped up. This feature helps technicians reproduce the issue when testing by mimicking the operating condition from before, making their jobs easier and more precise.

    For example, the "check engine" light pops up, and the engine computer stores that a misfire occurred. In this case, the driver might even notice symptoms like poor fuel economy and performance loss. Then, the misfire information that the engine computer stored will be available for the technician during special testing and evaluation, telling which driving conditions a misfire occurred, which caused the "check engine" light. Finally, the technician will use this stored information to know what kind of tests to perform.

  • Q: Are you driving with the "check engine" light on?

    We recommend you not to. The issue with driving with the "check engine" light on is that it is your vehicle's only engine warning light. So, if any other problem adds up to the car's engine or electrical system, you won't know because there won't be an extra warning; the light is already on.

    In this case, you may lead your vehicle to show more noticeable symptoms. For example, if a variable valve timing defect occurs, symptoms may include a rattling and ticking noise, power loss, fuel mileage drop, and oil leaks. You'll need to share these or other clues with your shop.

  • Q: Did you check if your gas cap is loose?

    A loose oil or gas cap can alert the computer system, which then notifies the driver with the "check engine" light. Fortunately, some modern vehicles even have a specific "check gas cap" alert. A loose gas cap has this impact because it allows evaporative leaks.

    If it's not a loose gas cap, the problem is elsewhere, and it's possible that your car shows these symptoms: no performance drop, raw fuel smell, and "check engine" light. In this case, you'll need additional testing to get your car past an emission test to renew its registration.

  • Q: Is your car slugging and having bad fuel mileage?

    If a catalytic converter isn't working correctly, the "check engine" light pops on the dashboard. Still, note that this does not mean the catalytic converter is broken; many parts working alongside it can sabotage and impact the converter's workflow. For example, engine misfires, ignition fuel, intake problems, and a failed mass air flow sensor can cause this type of issue. Common symptoms of this problem are poor fuel mileage, engine underperformance, surging, and hesitation.

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Vehicle Health Inspection Proof

Explore the following typical inspection results that show a potential cause for the symptom and select the one you believe is similar to your vehicle's issue.

Engine misfire

This image shows what engine misfires can involve. In this case, a faulty ignition coil damaged wires and connectors, causing misfires to happen more frequently. The mechanic found this during an underhood inspection.

This vehicle's driver came into the shop complaining about a "Check Engine" light constantly displaying on the dashboard. The technician ran a test drive and noticed low engine speed and poor acceleration.

The vehicle health inspection didn't lead to anything that could be causing the "Check Engine" light. At this stage, the technician needs to do a more thorough examination.

While performing additional tests with a scan tool to read live data, the technician found the engine's misfiring cylinder. After noting it, they then did more testing on different parts.

Typically, misfire investigations include testing of the vehicle's mechanical, ignition, and fuel system components. Parts such as spark plugs, ignition coils, and fuel injectors are common leads; worn spark plugs are usually the culprits.

After finding the faulty cylinder through additional testing, the mechanic needs to use a lab scope to track which cylinder component was the culprit and decide the next steps.

Does the issue look like this? if not accessible your shop will document it
Evaporative emission leak

This image shows a mechanic performing a smoke test. The test is vital for finding leaks in the emission system. In this case, the smoke seeps through a leak in the purge control valve.

This vehicle's "Check Engine" light was popping on the dashboard, and there were no driveability issues or performance drops. So, the driver brought it to the shop to track the issue.

The technician performed a vehicle health inspection and found no leads to any faulty components. Then, they did additional tests, including a computer scan, which read the vehicle's overall state and notified a small evaporative emission leak.

Evaporative leaks commonly concern a loose fuel cap, which wasn't the problem in this case since it was tightly put. So, it was something more complex. You can inspect the car for a faulty gas cap to avoid extra expenses.

The mechanic did a leak emissions test with a smoke machine to spot the leak's location. This procedure is part of additional tests that runs a smoke stream through the vehicle's system. Then, if the smoke seeps through anywhere, it's a possible leak point.

In this case, the smoke was seeping through the purge control valve. Still, it's valid to check the vent valve, evaporative canister, fuel filler neck, and gas tank since they're typical culprits too.

Does the issue look like this? if not accessible your shop will document it
Faulty airflow sensor

In this image, the mechanic holds a faulty airflow sensor. A broken sensor won't read airflow, causing the "check engine" light to pop up. Consequently, the computer can't properly adjust fuel mixtures.

The client came into the shop complaining about a "Check Engine" light alert on their car's dashboard. As part of an initial inspection, the mechanic performed a test drive, which showed a lack of performance and hesitation to accelerate the car.

Moreover, the customer also complained about a fuel economy loss since the "Check Engine" light first appeared.

The technician did a vehicle health inspection, which didn't lead to any faulty components. Therefore, the service will require additional testing. In this case, a computer scan is recommended.

During the scan test, the mechanic found that the engine wasn't receiving proper load. More specifically, it was getting 38% of the load required, resulting in a low-performing vehicle, causing it to lose fuel rapidly.

Moreover, the computer scan pinned that the mass airflow sensor was faulty. After performing individual tests on the airflow sensor, the mechanic concluded it was indeed damaged.

Does the issue look like this? if not accessible your shop will document it
Faulty catalytic converter causes the check engine light to pop up on the dashboard

This image displays an additional test consisting of scan-reading the car's engine computer system. The scanning device's screen shows the mechanic that the catalytic converter isn't operating.

This vehicle was constantly popping the "Check Engine" alert on the dashboard. The mechanic did an initial test drive and did not notice any driveability issues; the car was acting normally apart from the dashboard notification.

After performing a regular vehicle health inspection, the technician couldn't find any leads to the "check engine" issue. In addition, they also scanned the engine control module with a scanner tool and didn't find any other codes. An extra code would indicate another vehicle problem, and there weren't any.

Then, during an in-depth inspection, the technician checked both primary and secondary oxygen sensors; they were working well. But, when analyzing the engine's behavior, the mechanic saw misfires. At this stage, additional testing should indicate the misfire's cause.

Part of the additional testing for engine misfires is comparing the exhaust's temperatures before and after starting the catalytic converter. The result, in this case, indicated that the inlet temperature was hotter than the outlet. This behavior means that the catalytic converter is failing and requires replacement.

Now, the technician needs to investigate why the catalytic converter failed, requiring more complex testing on the vehicle's exhaust system. Remember that this issue can happen in any vehicle, including heavy trucks. For trucks, seek a certified truck mechanic to fix the issue.

Does the issue look like this? if not accessible your shop will document it
Lean codes

A detached vacuum hose was responsible for setting warning lights on the car's dashboard. In this image, you'll see how the mechanic spots it during an underhood inspection.

The customer came into the shop complaining that their car was showing a "check engine light" alert on the dashboard. In addition, the customer pointed out that the car was idling and acting odd.

The mechanic began the service by performing a test drive, which confirmed the car's idling behavior. They also spotted the car's maintenance records, noticing a recent oil change at a different auto shop.

During vehicle inspection, the mechanic opened the hood and spotted something odd on the air cleaner box; its vacuum line was out of place. This part serves as an air filter, and a disturbance on it causes unmetered air to sneak into the engine, which is terrible.

After finding the main culprit, the mechanic performed additional testing to clarify if this was the vehicle's only problem. Further testing consists of a computer scan with a scanner-like tool of engine control modules, code clearing, and fuel trims checking. This scan generates a vehicle health report for the shop's software to read based on diagnostic trouble codes.

Note that additional testing is more complex than a typical vehicle health inspection, and it's essential to rule out other damages. This might also mean that modern cars go through more in-depth inspections as they can have different types of codes.

Does the issue look like this? if not accessible your shop will document it
Faulty vvt

The variable valve timing depends on many things, which of these are clean solenoids and oil pressure. In this image, oil sludges were interrupting the variable valve timing by slowing down the process.

Firstly, VVT stands for variable valve timing, and it's a process of defining the timing of a valve lift event. This process happens during internal engine combustion.

The customer came into the shop complaining about a "Check Engine" light popping on their car's dashboard. The mechanic performed a driving test and found no driveability issues on the vehicle.

Next, the technician did a vehicle health inspection and noticed that the car's scheduled oil change wasn't up to date. This indicates a lack of maintenance, implying high risks for the car; the engine oil was black, which is critical.

This variable valve timing uses oil pressure to function correctly, and anytime oil sludges or dirty thick oil occurs in the system, it impacts the VVT. An oil change could fix the issue in the case we're discussing, but additional testing is essential.

Does the issue look like this? if not accessible your shop will document it

Typical Makes

That need Check Engine Light significantly more often than average vehicle makes

  • Make:


  • Model:

    Tacoma, Hilux

  • Engine Air Filter Box, Ignition Coil

Is this the make you are driving?
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Typical Fixes to Address the Cause(s)

The following chapters bases themselves on experiences from our auto repair shop; we'll describe related problems' causes and fixes.

"Vacuum hose replacement" fixes "Lean codes"

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Hey, Why Is My Car's Check Engine Light On?
Sometimes a problem is more challenging to describe than it initially looked like. If you are not sure your problem is described by this article, please find below similar vehicle symptoms, which might describe better the issue you are experiencing.