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

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

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 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 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.

How to Reset the 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.

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.

Question: what does code P1131 mean?

Answer: You may have a bad oxygen sensor - the one located before the catalytic converter. If you have a V engine, look for the one connected to the side where cylinder one is located. Also, other faults can set this code (lean-condition): a faulty MAF sensor, a vacuum leak, bad fuel system pressure, or a leaking fuel injector.

Question: what does the p0032 code mean?

Answer: This code points to the oxygen sensor (bank 1) in front of the catalytic converter. There could be a malfunction at the sensor, connector or circuit.

Question: Can you explain what P0300 and P0507 mean?

Answer: P0300 - Basically the computer has detected a random or a multiple misfire. The most common reason for this code to appear is when using worn out spark plug wires, plugs, issues with an ignition coil, oxygen sensor.

P0507 - the idle air control system (IAC) is running at high speed - (usually over 200rpm over the designated parameters). This may show up in vehicles with electronic throttle control or a related sensor(s). Also, common sources are vacuum leaks, IAC sensor or circuit faults, buildup around the bore of the throttle body, bad PCV valve.

Question: What does a P0016 code mean for my car?

Answer: The computer has trouble reading the signal from camshaft position sensor in bank 1. It might be out of alignment, a timing belt may have missing teeth - on the other hand there could be a restriction in an oil control valve passage.

Question: What does a P0170 code mean?

Answer: This code usually means the ECM (computer) can't control air/fuel trim as it was programmed to. The usual suspect is the O2 sensor (if five-years-old or older). But there could also be an air leak, vacuum leak, MAF sensor, or camshaft-crankshaft sensor issue.

Question: The check engine light has been on since the car was purchased 6 years ago. What do I do at this point?

Answer: If you haven't found any diagnostic trouble codes, the problem can be with a grounded light circuit. Try to fix the circuit; otherwise, check for trouble codes and try to fix any problems pointed by the codes.

Question: Can reducing the water coolant amount trigger the check engine light?

Answer: When the coolant level affects the operation of the engine coolant temperature sensor, it will trigger the check engine light.

Question: My check engine light came on and was steady (not flashing) for about a week without any apparent engine issues. One day I got in the car and before I put the key in the ignition there was a clicking noise inside the car and the check engine light went out. I still get the intermittent clicking. I have a 2010 Jeep Liberty, what could be causing this?

Answer: The clicking sound may come from a relay or switch in the fuel or emission system. Even when off, some systems depressurize at this time or do some other "work". Download the trouble codes form the computer and see what they tell you. Even if the light is off, there could be pending or history code.

Question: What is an po456 error code?

Answer: Definition: Evaporative Emissions System Small Leak Detected

Question: I need my CEL to come back on after accidentally being cleared on a scanner. Do you know how to do that?

Answer: I think the only way is to complete a driving cycle. You need to do some city and highway driving, accelerating and decelerating to have the computer go through the emission monitoring tests. A scanner tool will let you know when the computer has completed the tests. The trouble code, if the problem is still there, will come back on.

Question: How would you diagnose a P218F code after both the MAF sensor and EGR valve are replaced?

Answer: There are a few possibilities - If you replaced the MAF sensor because of a code, there might be a problem with the wiring or the connector.

Make sure there are no air leaks - air ducts properly sealed after replacing the MAF sensor, perhaps, triggering the O2 sensor to report a bad ratio. Check the fuel pressure and also check the O2sensor. If you have a V6-8 engine, check the O2 sensor for bank 2. Keep in mind that this is just a general description. It'll be a good idea to Google the code along with information about your specific vehicle because this could change depending on the model.

Question: What does a P0626 code mean?

Answer: This usually indicates a problem with the field circuit or the coil itself inside the alternator. You can see the coil through the vent holes on the alternator. The problem can be with a fusible link, a blown control circuit fuse, or the alternator itself has failed. Less likely, there could be a problem with the car’s computer.

Question: What should I do with these four fault codes: P0087,P0088,P0192,P0193?

Answer: Check the fuel system: fuel filter, pump, fuel pressure sensor, pump relay.

Question: Why didn't my1993 Camry trigger the check engine light when one of the injectors was faulty (causing a dead cylinder) and the rest of the injectors were dirty, running really bad?

Answer: Usually, a mildly restricted fuel injector would not cause the engine light to come on. However, if this same injector causes a misfire, the computer will notice it. If the misfire recurs, it will trigger the CEL. It the problem was intermittent at first, may there was a pending code stored, and the computer waited for the next test cycle. If it didn’t happen, the CEL didn’t come on. If you are sure there was a dead cylinder (not intermittent), probably there’s a problem with the ECM, but I’d check the EGR valve, and for good spark as well.

Question: What does a P1128 flashing code mean for my car?

Answer: Basically, a P1128 trouble code means the computer sees a difference in throttle position and intake pressure and the corresponding manifold absolute pressure (MAP) sensor. It's possible there's a problem with the sensor's circuit or harness connector. The sensor itself may be faulty. In a Dodge model, this could mean the computer is not able to go into close-loop mode. This can be tricky to fix. The problem can be in the fuel system, a bad MAP, a bad oxygen sensor, a thermostat problem or even a computer issue. You need to have this code correctly diagnosed. If too much raw fuel is reaching the catalytic converter, it can damage the device. The computer starts to flash the check engine light.

Question: My check engine light is on in my 2000 Pontiac grand am and blinks 2 or 3 times when I start the car. What is the problem?

Answer: Download the trouble codes and see what you get. When it initially blinks at startup, it seems like the computer is trying to go into a self-test diagnostic. This could be an issue with the test connector or the computer itself. You need a proper diagnostic to see where the fault is.

Question: My Kia Picanto high beams flash when I hit the accelerator, especially under load or when going uphill, and the check engine light comes on. I hear a click from the relay/fuse box area when this happens, and the engine does a brief stutter. I took it to Kia, and they couldn't solve it; I've been to an auto electrician, and they couldn't find fault codes or solve the problem either. I've also changed the throttle body and sensors. Do you have any idea what the problem could be?

Answer: It could be a fault in the alternator. When one or more diodes begin to fail, you get a sort of pulse in the electrical system when you step on the gas (AC sent from the alternator instead of DC). You should have the alternator checked.

Question: 2009 Chrysler Town and Country- Engine light flashes after idling in drive for about 30 seconds, then turns solid once I begin driving. In the past, the engine light has been solid for quite some time, and every time I have the diagnostic code run, it comes back to my gas cap. Any thoughts as too why the light flashes while idling, but doesn't flash while driving. Raw fuel to the Catalytic converter?

Answer: The gas cap may be loose or damaged. Unburned fuel going to the catalytic converter is what causes the flashing light. This usually comes from misfires. You may want to check for a leaking injector that might be flooding a cylinder when the engine is off. During normal driving, the cylinder may not have a problem burning the injected fuel.

Question: I have an 05 Honda Accord. The check engine light had been on for a while then cleared when the battery died. Car engine cranks but will no start. The check engine flashes then go out. The car has a new fuel pump, plugs, coils, starter and crank sensor. What could be the problem?

Answer: The problem could be in the ignition system. The check engine light will flash if raw fuel is getting to the catalytic converter to warn you that it can seriously damage it. Try scanning the computer again for trouble codes. If there are none, a few cranks will set the trouble code again. The code will help you track the problem. A sensor's circuit or sensor may be faulty and causing trouble in the ignition system.

Question: What does the p1130 code mean?

Answer: There may be a shorted swirl control valve (opened). Check the repair manual for your model to locate and test the valves. If you don't have the manual, your local public library may have it in the reference section.

Question: What does the poo68 code mean?

Answer: It's possible there's an air leak between the MAF sensor and the throttle body. Other possibilities are a faulty MAF sensor, clogged air filter, intake manifold leak. If there's an electronic throttle body, check it as well.

Question: What does the P0101 code mean?

Answer: The P0101 code points to an issue with the MAF sensor, either the circuit or the sensor itself. The computer is detecting the sensor's signal from the predetermined frequency. You might want to check if the sensing element is blocked or dirty); this is a common problem with MAF sensors. You can find MAF cleaning sprays at local auto parts stores if you need one. Otherwise, you may try trouble shooting the sensor using your vehicle repair manual. You can find Haynes manuals at your local auto parts stores or online.

Question: My ‘02 Mercury Sable has a misfire. The engine light flashes when I go up hills and right now it's flashing constantly. What could the problem be?

Answer: The misfire is allowing too much raw fuel to reach the catalytic converter. You need to find out the cause for the misfire before it destroys the catalytic. Pull down the diagnostic trouble codes (DTCs) from the computer memory. It may give you an idea about which system to start looking into. This other post may also help you.

Question: What can I do to resolve my van's trouble code P0430?

Answer: Have the oxygen sensors checked first. If they are OK, check the catalytic converter.

Question: I have a VW Jetta MK5 which has had a misfire issue for some time. I parked it for some time without repairing it and decided to start it recently. The check engine light started to blink and upon seeing that, I switched it off. When I re-started the car the light remained solid. What is the problem now?

Answer: You need to download trouble codes from the computer memory. The flashing light warns you of potential damage to the catalytic converter because of a misfire; if the light is now solid, the misfire could still be there, but other problem might have shown up. If necessary, you can go to a local auto parts store to get the codes downloaded.

Question: My check engine is flashing on my 2009 Chevy Silverado. What does that mean?

Answer: This usually happens when raw (unburned) fuel makes its way into the catalytic converter. The most common cause is misfires. The computer is telling you to fix the problem before it destroys the converter.

Question: The check engine light of my Toyota Corona was blinking when it reached the normal operating temperature of the engine. What would be the problem with my vehicle?

Answer: If the CEL was flashing, it probably was raw fuel reaching the catalytic converter. Usually, this happens due to misfires. In this case, it's better to fix the problem as soon as possible to prevent damage to the cat. This is what the flashing light is warning you about.

Question: Why is smoke coming from the exhaust and the check engine light blinking rapidly?

Answer: The engine may be burning too much fuel; check for a rich fuel condition. Download trouble codes from the computer and see what you get, and start from there.

Question: What does a P0601 code mean?

Answer: This is basically a problem with the computer itself. Have the PCM checked. If the problem is with the computer, sometimes it's possible to reprogram the computer; otherwise, it has to be replaced.

© 2015 Dan Ferrell

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