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Check Engine Light

Why does the check engine light come on and how do you reset it?

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2011-09-22 16:43:10
2011-09-22 16:43:10

The "check engine light" is a warning light that is illuminated when there is a problem with the EMISSION SYSTEM only. Emission system being the pollution control system. It is a good thing once you understand it.

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The " check engine light" is also known as a malfunction indicator light (MIL). You can disconnect the battery red lead for about 5 min and it will reset. However, if there is a hard fault, it will come back on.

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Disconnecting the battery won't do the trick on OBD 2 vehicles.

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Without a scan tool designed to reset the electronic control module (ecm) a do-it-your-selfer can't reset the check engine light.

Pull the codes from the computer using an OBD (II) code reader. Match the code to the troubleshooting procedure, follow the procedure to find the source of the fault. After you have repaired the fault, the light will go out if that was the only problem. There are "monitors" or self tests the computer runs the car through a drive cycle, if a problem occurs, it may not run all of the self tests until that problem is taken care. Therefore, another problem may exist.

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One point that was brought up a a recent meeting of technicians was that the amount of hydrocarbons is greater when the gas cap is left off than when the engine is running. Hydrocarbons are part of pollution emitted as gasoline evaporates. Going a step farther, one facet of the emission system is the "Evaporative" portion. This is when the fumes from the gasoline are leaking from the system into the outside air. This is one part of the emission system that can trigger a check engine light. I would say that about 7% of the vehicles that have a check engine light are the result of a loose or inadequate gas cap.

But understand that many scenarios are possible with the "check engine light" The vehicle's powertrain computer (note that some vehicles have 17 different computers) will run a series of self-tests. They will only run under certain criteria and they can be vastly different from manufacturer to manufacturer. Some self-tests are not run until preceding ones have run successfully. So if there is a problem in one particular area that is preventing another self test from running, you can have a situation where one problem is fixed, but another still exists. If you fix a problem and drive the car through a drive cycle that sets the monitor (or self test) the light will go off as it passes that criteria that triggered it in the first place.

After 1996, the auto industry went to a idea called OBD II (On Board Diagnostics). This was to get all the manufacturers onto a similar plane for troubleshooting and powertrain control. While they still differ vastly, many corrections and adaptations were made for technicians to better fix the check engine light problems.

Prior to this there were so many different and poor troubleshooting data from a check engine light problem that resolving the problem was much more difficult.

Many early warning lights of this nature were set to illuminate based on mileage. An oxygen sensor was one of the things that was meant to be replaced when that mileage was hit. This is much like many current "Change oil lights" which are set up to come on after a pre-set mileage.

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Great question and good answer above. There is one thing to add however. On my 2004 Buick for example I am required to manually reset the "check oil" light myself every time I get an oil change. This ensures that the system understands the condition the oil is in. If/when I do not reset it, not long after my expected date the light goes on by default as a reminder.

Your instruction manual would outline this. Mine for example is simply...

1. Insert key

2. Turn key to ON, without engaging engine (just before the engine cranks, and leave key there)

3. When placed to ON, press and depress the brake slowly 5 times in 5 seconds.

4. Turn key to off.

5. Turn key and start engine.

The check engine light is now reset, if it returns shortly it is an actual issue, rather then a reminder.

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Some Ford Mondeos have a 'Service interval expired' warning light. It is pretty much the same as the "check oil" light mentioned in the answer above.

On that type of Ford Mondeo there is a small hole in the display screen in front of the 'Service interval expired' warning light. To reset it, just turn the ignition key until the all warning lights come on but do not start the engine. Then press a tiny button in the warning light display panel by poking a straightened metal paper clip through the hole. If you hold it steady for 10 seconds whilst keeping the ignition on, the warning light goes out. It will come on again when the service time period again expires.

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Related Questions


No, it may or may not reset the check engine light. If you did not repair the problem that caused the light to come on it will just come back on.



The check engine light does not come on for an oil change.


The check engine light does not come on when an oil change is due. The check engine light is for an emissions related failure. It is reset with a scan tool after the fault is repaired.


Have the problem that caused the check engine light to come on repaired. They will reset the light with an OBD2 scan tool. The light will just come back on if the problem is not repaired.


The Chrysler Cirrus did not come out until 1995. You reset the check engine light with a scan tool or by unhooking the battery. If you have not repaired the cause of the code the light will come back on.


No, it is not illegal to reset the light. It is however illegal to remove the bulb so the light will not come on or to in any other way prevent the light from working as it should. If you do not repair the problem that caused the check engine light to come on in the first place it will just come back on after it is reset.


To reset the Check Engine Light you must go to the trunk of your car get a cro-bar and smash the dash board until the light doesn't come on no more...


use a code reader to check the problem and it will reset the light, but if you do not fix the problem the light will continue to come back on.


By repairing the problem that caused the Check Engine light to come on in the first place. You can then reset the CE light with a scan tool. Reset the light without repairing the problem and it wll just come back on.


The check engine light on a 1999 Cadillac DeVille can be reset by disconnecting the battery for 30 seconds. The light will immediately come back on if the problem still exists.


The check engine light on a 2003 Honda Accord will be triggered when something on the engine is malfunctioning. To rid the check engine a light the ECU will need to be reset.


If the Check Engine light is on then there is a problem with the emissions system. Once that is repaired the light can be reset. Reset it without repairing the problem and it will just come back on.


If the Check Engine light is on, then you have a problem with the emissions system. Once the repair is made the light must be reset with an OBD2 scan tool. If you do not repair the problem the light will just come back on.


Have you repaired the problem that cause the light to come on in the first place? If not the light will just come back on. You reset the light with an OBD2 scan tool.


No there is not a reset button. You can try disconnecting the negative battery cable for 5 minutes and that may reset the check engine light. If not it will have to be reset with an OBD2 scanner. Remember if you do not repair the problem that caused the light to come on you are wasting your time resetting it as it will just come back on.


You can disconnect the negative battery cable for 5 minutes which may or may not reset the check engine light. If that does not work you will need to have it reset with an OBD2 scan tool. Know that the light will just come back on if you do not repair the problem that cause the light to come on in the first place.


I think you are assuming that to reset the check engine light you can just disconnect the battery. Disconnecting the battery on a 2004 OBD2 compliant vehicle will not reset the check engine light. It must be reset with an OBD2 scan tool. This would work on OBD1 vehicle but not on OBD2 vehicles. So the radio issue is mute. Have the problem that set the check engine light repaired and then reset the light with the tool. Reset the light without correcting the problem and the light will just come back on.



First of all you must repair the problem that caused the light to come on in the first place or it will just come back on after being reset. You can try disconnecting the negative battery cable for 5 minutes. This may or may not reset the check engine light. If it does not then you will need to reset it with an OBD2 scan tool.


Have the problem repaired and they will reset the light. Resetting the light without repairing the problem and it will just come back on. You reset it with an OBD2 scan tool.


If you don't fix the problem that is causing the check engine light to come on you can unplug the battery a thousand times and the light will continue to come back on. Have the codes pulled and find out why it's turning it on in the first place.


You need a code reader to reset it or go to a shop and get it reset. Average $20 however, if you don't fix the problem why the check engine light came on in the first place it will only come back on again.


Did you reset the light? I don't know what gasket you changed, but undoubtedly that was not the problem. The check engine light comes on when there is a problem with the emissions system.


You need to borrow or purchase an OBD2 code reader with a check engine light reset. (most code readers have it) But bear in mind that if you don't solve the problem that set the code, the check engine light will come back on shortly.



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