What it Means When a Check Engine Light is On or Flashing

Updated on April 1, 2016

The check engine light is part of your car's on-board diagnostic (OBD) system. This system is operated by the electronic control module (ECM), which is your car's computer. In modern vehicles, the ECM controls almost every major electrical or electronic system and continually scans for out-of-range operating parameters as reported by numerous sensors and actuators.

For example, a sensor or actuator may detect a misfire, poor fuel injection, unusual output voltage to the secondary ignition system, erratic operation of the fuel pump, or any other condition that might affect engine performance or emissions. Your car's computer will first try to correct the problem or wait a number of cycles. When the computer can't correct the problem, and it doesn't correct itself, the OBD system will store a diagnostic trouble code (DTC) in memory. This triggers the check engine light.

Since the computer stores a specific trouble code identifying the particular malfunction, it is a way to help car technicians and owners determine the nature of a problem and potential system or component involved. With the right tools, it also helps you troubleshoot your vehicle whenever the engine light illuminates.

In this guide you'll find out what type of problems turn on the check engine light, what it means when it flashes, how you can retrieve the trouble codes stored in your car's computer, and how to go about deciphering those codes to help you fix your vehicle.

Five Top Check Engine Light Causes

Check these first:
• Broken wires
• Torn vacuum hoses
• Loose vacuum hoses
• Loose electrical connections
• Engine mechanical problems

What the Check Engine Light Does

Since 1996, motor vehicles have been equipped with the OBD-II (second generation) diagnostic system, an enhanced and standardized version of the previous generation. The OBD system—as it relates to the check engine light—monitors and controls the amount of emissions produced during the operation of your car by constantly checking and correcting input values to the appropriate systems with the help of sensors and actuators, including:

* Engine coolant sensor
* Oxygen sensor
* Knock sensor
* Camshaft and crankshaft sensors
* Evaporative emissions (fuel vapors) sensor
* Air meter sensor
* Canister purge solenoid
* EGR solenoids
* Fuel injectors
* Fuel pump
* Idle air solenoid
* Idle speed motor

Whenever the computer detects an abnormal condition through one of these, it sets a DTC in memory and lets you know by illuminating the light.

A check engine light warns you about emission-control related problems.
A check engine light warns you about emission-control related problems. | Source
A check engine light helps you keep emissions under control.
A check engine light helps you keep emissions under control. | Source

What a Flashing Check Engine Light Means

Intermittent Flash
Continuous Flash
The light flashes only when the problem appears. Possible causes include loose connectors or broken wires that disconnect when car goes over bumps. Problem may not be critical.
A critical problem has been detected. Causes may include fuel passing into exhaust manifold.

Intermittent, Continuous or Check Engine Light Flashing

When a potential emissions-related problem warrants your attention, you'll see the light illuminate in one of three different patterns.

  • Intermittent

Sometimes, the problem comes and goes. This is referred to as an intermittent or soft failure and can cause the light to flash, stop flashing, and then start again. The problem appears only when certain conditions appear. For example, the loose connector or broken wire that keeps connecting and disconnecting when the vehicle travels over bumps or irregularities on the road, causing the light to turn on and off for periods at a time.

  • Continuous Illumination

Whenever a problem causes the light to come on and stay illuminated, a hard failure is present. A dirty or failed mass air flow (MAF) sensor, for instance, will remain out of its normal operating parameters and cause the check engine light to remain on until you clean or replace the sensor.

  • Continuous Flashing

When the check engine light flashes continuously, it means you have a critical problem that will cause serious damage if neglected. Usually, this originates in a misfire that allows fuel to pass into the exhaust manifold and down to the catalytic converter where high temperatures ignite the fuel. Eventually, this condition will damage the exhaust system or catalytic converter. You need to fix the problem soon before a minor repair turns into a major and expensive one.

A check engine light flashing prevents you of critical problems that will damage the catalytic converter.
A check engine light flashing prevents you of critical problems that will damage the catalytic converter. | Source

Common Reasons for Check Engine Light

The light will flash if the computer detects:
• Engine mechanical problems including worn parts.
• Fuel system problems, bad injectors, regulator, or fuel pump.
• Increased emissions, bad oxygen, EGR valve, or EVAP sensor.
• Vacuum leak, hose, or gasket problems.
• Actuators or sensors sending bad signal to computer.
• Computer malfunction.
• Electrical short in a monitored circuit.
• Electrical connection problems, corrosion, or broken wires.

Check Engine Light Codes

When the computer detects an abnormal condition, it will store a DTC in memory and turn on the check engine light to let you know of the situation. Let's say, for example, that the oxygen sensor in the exhaust system detects too much oxygen in the exhaust stream and sends this information to the computer.
The computer tries to rectify by injecting more fuel into the engine. However, if one or more fuel injectors fail to respond, the computer will detect the problem, store a trouble code, and turn on the light.

Manufacturers have standardized the codes across all 1996 and newer models. This means that a specific code will point to the same type of problem regardless of what make and model you own. This makes it easier to troubleshoot and fix your car.

Diagnostic codes contain alpha-numeric values. The configuration of a DTC begins with a single letter, followed by four digits. So, a typical trouble code looks like this: P0102.

  • The first letter denotes the type of code. This could be a "P" (powertrain), "U" (wiring-network), "C" (chassis), or "B" (body). We're concerned with "P" codes here, since they relate to the check engine light.
  • Next, you'll see a "0" or a "1" digit: A "0" means you are dealing with a generic OBD code found in most vehicles; a "1" means you have a code specific to your car make and model, better known as a manufacturer specific code.
  • The next digit will tell you in which system the computer detected the problem. A "1," for example, means the problem originated in the fuel and air metering system.
  • Finally, the last two digits identify the specific circuit or component involved and the nature of the problem. For example, a "02" means the computer detected low input in the mass air flow sensor circuit.

Retrieving Computer Codes

Once you see the check engine light illuminate on your dash panel, you can retrieve the trouble codes.

  • OBD-I Codes

On pre-1996 vehicles, you'll find different procedures to retrieve the codes. With some models, for example, you can use a code reader—available at most auto parts stores—or an analog voltmeter. Other models use the ignition key to cycle between the on and off positions a number of times to read the code blinking light on your dashboard. You'll find the particular procedure in the repair manual for your specific make and model. You can buy a manual at most auto parts stores or online.

  • OBD-II Codes

Retrieving codes from a second generation OBD system requires the use of a scan tool. Scan tools come in various degrees of sophistication. You can spend less than a hundred dollars or a few thousands dollars on one of these tools.

For most owners with cars equipped with OBD-II systems, $70 and $200 dollars will be within their budget to afford a scan tool good enough to retrieve most check engine light related trouble codes. You can buy quality, relatively inexpensive scan tools at most auto parts stores or online. Some will display the code and a brief description of the code. Others will only give you the code, but most come with their own instructions manual and a list of diagnostic codes and their definitions. Additionally, many websites will help you define the trouble code and give you additional help in translating the code into a troubleshooting procedure.

Still, you may not have to buy a scan tool to get the codes. Some auto parts stores will run a diagnostic on your car for free. They'll hook up the scanner to your car's computer and retrieve any stored DTCs their scanner can find.

Yet, as useful as DTCs are to troubleshoot car systems problems, you need to keep one thing in mind when working with codes: Your car's computer will only point toward the circuit or component involved in a potential problem and the type of problem it's dealing with. This means, the codes don't give the whole picture. You still have to confirm the circuit or component has failed.

One of the main reasons is that a fault on a wire, hose or connector, for instance, can fool the computer—and you—into thinking that another component has failed.

Take, for example, a loose or torn vacuum hose leaking vacuum. The oxygen sensor will report to the computer that fuel rate is too lean. However, unable to adjust the fuel ratio, the computer will set a "dead oxygen sensor" trouble code instead. Obviously, replacing the sensor won't fix the problem. So you need to troubleshoot the oxygen sensor to verify that in fact the sensor has failed. Otherwise, you'll start replacing good components without actually fixing the problem.

Retrieving Trouble Codes

How to Reset the Check Engine Light

Once you've diagnosed and repaired the fault that caused the light to come on, you need to clear the trouble codes from the computer's memory. On older vehicle models, you disconnect the car battery for a few seconds to clear the codes from memory.

But don't use this procedure on newer models or you risk erasing the computer system adjustments as well.

Disconnecting the battery on some OBD-II systems will require the computer to relearn sensor inputs—for days or weeks—and adjust output parameters to configure a driving strategy for better engine performance. Also, you'll erase your radio and alarm settings. In the meantime, you may notice the engine not working quite "right." Besides, the ECM on some modern vehicles can keep trouble codes in memory for a few days even without battery power. Instead, on a 1996 or newer model, use the scan tool to reset the light.


Disconnecting the battery on some OBD-II systems will erase the driving strategy from the computer's memory.

Whenever the Check Engine light on your dash panel comes on, try to find out the reason for it as soon as possible. Self-diagnostic systems have become more advanced, going beyond typical monitoring operations. Modern OBD systems not only report system problems, but also check how efficiently a system and its components are working. The computer will let you know when one of these components is about to fail, saving you some money and road headaches in the process if you take advantage of these features. Even more, when a potential failure can cause extensive damage, the OBD system will produce a flashing signal to help you avoid expensive repairs.

Test Your Knowledge of Check Engine Light Problems

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    • Dan Ferrell profile image

      Dan Ferrell 2 weeks ago

      Hi BD,

      It's possible the canister purge valve is leaking and passing fuel fumes to the engine. This may be causing the flashing light - need to have it check, the catalytic converter may be damaged.

      Good luck

    • profile image

      BecD 2 weeks ago

      Hi there Dan!,

      My sisters 2002 Holden Astra is turning over but won’t start. Her check engine light is flashing and it sounds like her carbon canister valve is ticking?! Pls help.


    • Dan Ferrell profile image

      Dan Ferrell 2 weeks ago

      Yes 10 minutes won't do much harm. Probably what's happening is that some unburned fuel is passing through the exhuast system and the computer is warning about potential damage to the catalytic converter if you don't fix it soon.

    • profile image

      Marie 2 weeks ago

      Hi, my check engine light starting flashing today, mechanic said to bring it over in the morning, can I drive it to get to my mechanic who is 10 mins away.

    • Dan Ferrell profile image

      Dan Ferrell 7 weeks ago

      Hi MS

      There may be an issue with the circuits. Check for loose connections, damaged to wires.

      Good luck

    • profile image

      mohamed safy 7 weeks ago

      my Octavia 2003 has auto G B .CHECK engine constantly illuminating.equippedwith bad shifting G B . i had replaced 3 times sensors GEAR BOX G38 &G68 .AFTER CARING ABOUT 500KM IT illuminates

    • Dan Ferrell profile image

      Dan Ferrell 2 months ago

      Hi vijaikumar

      Did you get the DTCs (trouble codes) from the computer? See if they point to an ignition, emissions system failure first.

      Good luck

    • profile image

      vijaikumar. L 2 months ago

      Hai sir,I am in India,the Nilgiris. My opel astra 1998 model was didn't start no spark. The engine management light was blinking. I change ignition coil pack and wires and cranksensor. I need your help.

    • Dan Ferrell profile image

      Dan Ferrell 2 months ago

      Hi Dalton,

      You may have problems with the shift solenoid or even the battery, make sure it's fully charged and in good operating condition. Usually when the CEL comes on in a situation like this is to tell you that the engine is in safe-limp mode to prevent damage.

      Good luck

    • profile image

      Dalton 2 months ago

      Hi Dan, my car won't gear up when hitting the highway. I accelerate but will stay around 40-50 mph. When this happens an intermediate check engine light appears. I can slow down and the car will jump into the next gear though. I drive a 2004 Grand Prix.

      Any suggestions are appreciated!

    • Dan Ferrell profile image

      Dan Ferrell 3 months ago

      Hi Ronny

      The blinking light means there could be damage to the catalytic. Get the DTCs stored in memory and take it from there before you start changing parts.

      Good luck

    • profile image

      Ronny 3 months ago

      Hi, My Honda Accord 2009 LX has a constant blinking check engine light on and i keep hearing a ticking sound from the engine. Also i noticed the car is not firing properly and the smoke from the exhaust is black . please what should i do to fix this.

    • profile image

      Mark Boontjes 4 months ago

      Hi Dan thanks for your reply and input, suspect the same as you have mentioned and have checked most of the wiring harness and connections with no luck, no error codes with OBD scan. Since my post I checked again and as said lightly touched the wires to the indicator/high/low beam switch (Yellow/white wire this time) and since all is working fine again till next time, so far the engine check light comes on every time and starts fine even the throttle control valve is working fine now again which was a problem before. My suspicion is that there is a problem with one of the wires in the loom under the dash. Will only be able to check the wiring again when the problem comes back. As you say intermittent problems are very hard to trace, this one more so as I have to check continuity through each wire which could end up not being a true reading as by touchin the wire continuity could be found.

    • Dan Ferrell profile image

      Dan Ferrell 4 months ago

      Hi MB

      Intermittent problems like those are hard to trace. But since you mention that sometimes the car won't start, I'm wondering if there's a bad ground.

      Have you checked the battery and engine grounds? Just a thought.

      Also, have you scan for codes, just in case?

      Good luck

    • profile image

      Mark Boontjes 4 months ago

      2002 Opel Corsa light 1.4, my problem is somewhat different though, when I switch car on and the engine check light comes on (it blinks) then the car will start, but then sometimes the light won't come on and the car won't start. To my thinking it must be a weak or bad connection somewhere because if I fiddle with the wiring it comes back on, sometime a light touch other time more vigorous, but as it goes never the same wiring loom or wire. How now, has anyone experienced this before.

    • Dan Ferrell profile image

      Dan Ferrell 4 months ago

      Hi Bill

      Have you checked for DTC (trouble codes)? Usually, when the CEL flashes it means trouble for the catalytic converter. Check for pending codes. Also test the MAF and oxygen sensor.

      Take a look under the hood for loose wires or vacuum hoses. And make sure the air cleaner ducts are sealed, not allowing unmetered air through.

      Good luck

    • profile image

      Billy 4 months ago

      So my check engine light was consistantly on but when I accelerate it started to flash but when I unhooked my mass air flow sensor it didn't flash any more

    • Dan Ferrell profile image

      Dan Ferrell 4 months ago

      Hi LR,

      There's probably some damage to the catalytic converter. Take the car to the shop for an inspection.

      Good luck.

    • profile image

      Lise Rossignol 4 months ago

      I drove into a bump then the engine light start to flash and the car started to jurk when I was going slow and there is a weird smell coming out of my truck. Avalanche 2005 1500. My husband changed the spark plugs and now the light stays on for a while then start to flash while driving. And still runs funny and have that smell.

    • Dan Ferrell profile image

      Dan Ferrell 6 months ago

      Hi Tad,

      You may have more than one issue going on. The battery light usually points to a problem with the alternator or charging system. Make sure the belt is correctly adjusted, and have the alternator checked if necessary -most auto part stores will do it for you.

      The engine light blinking is telling you a serious problem is affecting the catalytic converter and may cause damage.

      Scan the computer memory for trouble codes (DTCs). Start there. This may solve the engine stuck in high gear, but test the idle control valve, if necessary.

      Good luck.

    • profile image

      Tadpole 6 months ago

      My battery light is on engine light blinks car act like it stuck in high gear

    • Dan Ferrell profile image

      Dan Ferrell 7 months ago

      That certainly is new. Who would've guessed - system too lean and the culprit was a bad spark plug. I'd think this would produce a misfire code. Go figure.

      Thanks for the comment Terry.

      Good luck

    • profile image

      Terry 7 months ago

      Well, here's a new one for you. The engine light and stabilitrak/traction control lights came on after new spark plugs were installed. Engine was jerky (hesitation) and brakes did a pulse grab occasionally when applied. Error codes were P0171, P0174 and a G??? code for the Traction Control. Guess what?...the problem was not the air flow, oxygen or fuel system sensors or the fuel injectors or vacuum lines, etc., ...it was one of the new spark plugs (GM iridium?) that was faulty. How about that! New plug replaced, running like silk. The vehicle is a Saturn Outlook 2007 with 135,000 Km that is in showroom condition. Always well looked after. Hope this helps.

    • Dan Ferrell profile image

      Dan Ferrell 8 months ago

      Hi David,

      You might want to check the speed sensor(s). If it/they failed, the ECM detected the fault and it may cause all bunch of lights to go crazy. Double check the trouble codes, make sure the you don't have also a catalytic failure (blinking).

      Good luck

    • profile image

      David 8 months ago

      My 1996 Prado speedometer and odometer stopped working. When the speed increa, the O/D Off light started blinking on the dashboard. I did the manual diagnostic using a jumper inserted between TE1 and E1. The check engine light continuously blinking steadily. What does it mean?

    • Dan Ferrell profile image

      Dan Ferrell 9 months ago

      Hi RL,

      You seem to be getting a warning for the catalytic converter, but it's rare it doesn't show any codes. If you can, check the pre-catalytic O2 sensor and the post O2 sensor. It's possible the post-one is failing but I doubt it. Check it as soon as you have a chance to prevent damage to the converter.

      Good luck

    • profile image

      Rodney Lacy 9 months ago

      Check engine light will blink for several minutes or stay on. I have used a code checker but it will not show a code, just blink as long as the checker is plugged in. 2000 chevy silverado 1500 5.3.

    • profile image

      Steve w. 17 months ago

      This write-up has been very informative. it is written so even dummies like me can have a little knowledge of what they're looking for. My trouble where my vacuum lines were Rotted .Thank You AxleAddict.