Tip & How-To about Vehicle Parts & Accessories

Check engine lights and reading and defining the codes

If you have a check engine light and you've had the car scanned with an OBDII (OBD2) code reader. You can save yourself a lot of time by looking up the code(s) on google. simply search OBDii code Pxxxx (your vehicle eg. 04 tahoe) you may have to look through a few search responses until you find the info you are looking for, but it will save you a lot of time and headaches. If you can't find what you are looking for, or you are having trouble figuring out what it means and you are going to post a question on this site, be sure to include year make and model of your vehicle because some codes are vehicle specific and that information could be crucial to getting the right answer, it will also help the experts out if there is a known or common issue associated with your vehicle that would cause that code to appear. If you want/need more immediate help call your local AutoZone. Their computer system has nearly all possible OBDii code definitions listed, as well as 3-4 possibilities of what is causing that code to appear, while it may not tell you exactly what is wrong, it will/should at least point you in the right direction. Also keep in mind that unless you had your codes read with a more sophisticated diagnostic code reader (like the $2000 unit they use at most garages) there is no guaranty that reading the code alone will tell you how to fix the problem. Most of these code readers are designed simply to read the code that your on board diagnostics system is communicating, tell you what the code means, and give you a general idea of what could be causing it.

A few things to remember when dealing with check engine lights. First off; while your check engine light may not always be on for a serious problem, it is never a good idea to ignore it or wait a long time to have it fixed, some problems could go unfixed for long periods of time without causing you to break down, but letting it go for too long could cause other problems to occur.
Next: Neither reading the code, nor just turning off the light will actually fix your problem. In some cases your car's on board diagnostics may have detected a problem that was caused by the weather or outside temperature, or some other condition that can cause a code to appear without an actual "repair needed issue" being present, and may clear itself of that code during a future "sweep", if this happens, there is really no need to worry about it, unless the car was running rough, or performing poorly while the light was on, in that case you should have it checked out just to be sure, but if the light has gone out, scanning it wont do any good.
If the light is out, there are no codes to be read. If your check engine light was on this morning, and turned off an hour ago, a code reader will not be able to tell you why the light was on, they are only able to get codes from the car when the light is on signifying that trouble codes are present. which brings me to....
There is no code reader/diagnostic computer that can be hooked up to your car to tell you exactly what is wrong with it. Most of these systems are designed to pull the code from your on board diagnostic system to point you or your auto technician/mechanic in the right direction towards repairing the problem. Some of these machines can read "live data" which is the data readout of many of your car's sensors, it takes proper knowledge to know what these readouts mean and while they may not pinpoint the exact problem, a well trained technician or someone with plenty of experience can use this live data to get a better picture of what is going on with your vehicle.
When you get a check engine light and you have the code read, don't just assume that what the code says is actually what is wrong with your vehicle. To give you an example of what I mean... I've been working in the Automotive business for over 10 years and I've scanned many MANY check engine lights in that time. As most people that have had their engine lights checked may know, one of the most common codes to come up will be a code related to an Oxygen Sensor, when these codes come up it is almost NEVER the oxygen sensor that is the problem. Also don't just assume that because your CEL is on that it will be an oxygen sensor code, it could be an oxygen sensor code or it could be something else entirely.

A few final thoughts. If you replace a part based on what your codes were telling you, and the light goes out and stays out but comes back on with the same code, you're in one of two situations, first: the code was misinterpreted and the problem was misdiagnosed. The reason the light went off and stayed off for a time is because you introduced a brand new part to the system, that part was most likely brand new and running at a more efficient level than everything else on your car causing your vehicle to temporarily run as if the problem has been solved, once your new part has calibrated itself to your vehicle, the problem will resurface. This doesn't always mean that your new part went bad already, it just means that you and your car both thought that the new part fixed the problem and after a short time your car figured out that it didn't actually correct the issue. On that note, car parts are man made, they are all capable of premature failure, including failure right out of the box. It doesn't matter if the part was made in China, Mexico, or USA; it could last 10 years or 10 seconds if even one of the components was bad when the part was made, or if the person that assembled it made even a minor mistake, it happens that's what warranties are for. Lastly, back on the topic of oxygen sensors. If you are going to replace one, replace them all. Oxygen sensors communicate with each other and with the engine computer, if one is bad and is reading wrong it could and in a lot of cases, will cause the rest to go bad simply from overcompensating for the incorrect readings, they may not be bad at the same time but once one goes bad the rest will very likely fail as well, it is always best to replace them all at the same time so that they are introduced to your vehicle while they are all in the same condition. The same goes for catalytic converters, if you replace your "cat", replace your oxygen sensors at the same time. Malfunctioning oxygen sensors can wreak havoc on your "cat" and they are too expensive NOT to replace the parts that are the leading cause of their failure...

Take my ramblings for what they are worth, I am ASE certified and I've been doing this stuff every day of my life for the last 10 years, I'm not always right, but it has happened once or twice. Have a Great day and good luck out there !

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2 Answers

check engine light on, can it be turned off without taking to mechanic shop. 2006 750li bmw

autozone and others will check and report for free. they wont clear but you should know the code anyway. you can get your own OBD2 reader on Ebay for about $20

Jul 17, 2014 | 2006 BMW 750 Li

2 Answers

Is there anyone know how helpful Code Reader tools are?

I actually use a cheaper one I picked up from Mac tools that sells for $60 that I use as a professional. I like that xcardiag as a pro tech but unless you really read the book and use the tool to it's limit.. I think a cheaper scanner may be better for a xcardiag who's just looking to check and clear codes.

The codes are also in many places online, including the manufacturer codes that these tools read but can't display what they are as each manufacturer may call the code a different name.

CAN means CAN network.. or basically think of the first odbII information being sent to the scan tool and between modules as dial up.. Can is like say DSL.. it's quicker but a lot of the test tools can't pick up this higher speed info and most cars started around 2003 using the newer Network speeds.

Depending upon how educated you want to get on diagnosis using a scan tool.. Id say the xcardiag may be a little more advanced than the normal may need... as my $60 MAC tools one can basically do what the xcardiag does but is harder to use than their tool... that and this xcardiag basically has the video to teach the DIY about OBDII whereas mine is aimed as a cheap scan tool for professionals.

The freeze frame is what I use as a pro tech with the code.. it tells me what was happening when the code set and why. If you educate yourself... a cheap scan tool like this is great and you can diagnose most OBD II issues.

So depending upon your willingness to learn about OBDII and how much you gonna work on vehicles, should help the decision better.

Just take a look :


May 10, 2012 | Cars & Trucks

1 Answer

My 2001 Ford Crown Victoria has had the check engine light on for a while, and the code came up as something to do with the fuel pump circuit. Now I've been seeing some sluggish acceleration and a kind of jerking when I accelerate/cruise. Also, when I accelerate and the headlights are on the left turn signal indicator flashes faintly. I've heard I might be able to replace the fuse, or something to that effect. Would I find that information in the manual I've got, or should I invest in a more complete manual? Or should I just bring it in?

Take it to a repair shop & then use the
computer to study OBD2 Systems & get
a cheap code reader

Look at a fuse,they do not fix anything
& hardy ever blow.

When you say check light has been on for awhile
Are you not aware of why the dash light come on?
They have been on all vehicles for 31 years now

Any dash light is a System Malfunction,& you need to
check for codes within a couple of days or stop using
the car,each & every time,unless you understand the
current OBD2 Systems & have at least a code reader

Jul 02, 2011 | 2001 Ford Crown Victoria

1 Answer

check engine light came on 05 altima

If you are a DIY'er you can get a OBDII scan tool that connects to your laptop that can tell you the error code and allow you to reset your check engine light. This will save you $80 each time over going to a mechanic for a diagnostic. They are easy to use and will help you know what the issues are if you do have to go to a mechanic.
Check out http://www.fitityourself.netau.net/OBDII.html
Sometime the check engine light comes on if you did not fit your gas cap tight enough. You need a code reader to find out the error code to help solve your problem.

If it is something simple (like a gas cap) and you do not want to get a code reader to find out what codes (problems) your vehicle computer is saying there is. You can try disconnecting the negative battery lead for 5 minutes. This should reset your check engine light. If the light comes back on, you have a problem and can take it to a mechanic or buy yourself a code reader.

Feb 16, 2010 | 2005 Nissan Altima

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