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What It Means When a Check Engine Light Is On or Flashing

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

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.

The Top 5 Check Engine Light Causes

Check these first:

  1. Broken wires
  2. Torn vacuum hoses
  3. Loose vacuum hoses
  4. Loose electrical connections
  5. 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.

A check engine light helps you keep emissions under control.

A check engine light helps you keep emissions under control.

What a Flashing Check Engine Light Means

Intermittent FlashContinuous 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.

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


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.

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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.

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.
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.

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 thousand 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.

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 checks 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!

For each question, choose the best answer. The answer key is below.

  1. This sensor won't trigger the Check Engine light because of a hard starting problem but the others will:
    • Faulty mass air flow sensor
    • Defective coolant temperature sensor
    • Faulty oxygen sensor
    • Bad fuel injector
    • Bad crankshaft position sensor
    • Bad idle speed motor

Answer Key

  1. Faulty oxygen sensor

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: The check engine light of my vehicle is continually flashing. What does this mean?

Answer: Some raw fuel is reaching the catalytic converter and could damage it. You need to make this repair before you have to replace the cat.

Question: About a couple of months ago my check engine light was on, and it was due to engine misfiring on all four cylinders. So I replaced the plugs and the ignition coils plus changed the oil and filter. But the light still stayed on. So now the light flashes and my car won’t accelerate and my engine is making a loud noise and moving a lot. What is the problem?

Answer: The check engine light is flashing because the catalytic converter could be damaged. There’s seems to be too much raw fuel going through he exhaust. One or more sensors may be sending faulty signals. Download the trouble codes and see if you can find some information that can point you to a specific component(s) or system.

Question: My engine light is blinking in my 2008 Ford Edge. What's the problem?

Answer: Usually, this means the computer is warning you about raw fuel reaching the catalytic converter. This could be a misfire issue, but you need to diagnose the problem first. Do it as soon as possible before serious damage is done to the cat.

Question: The check engine light is on and when it is reset no error code comes up, what does this mean?

Answer: If the engine light comes on and you find no codes, there might be a manufacturer OEM code that many generic reader or scanners won't read (unless there's something wrong with the reader). Another possibility, but not as common is a problem with the computer or related issue. Your mechanic most likely can diagnose the problem. Take the car to an auto parts store and have them download the codes for you and see if they can get any.