How to Read Codes From Your Check Engine Light (For older cars)

You can pay a mechanic a bunch of money to read your car's error codes when your check engine light comes on, or you can grab a paper clip and read the codes yourself.

After 1996 all cars were made to hook up to an external error code reader. Before 1996 however, cars reported their errors from the computer themselves via a flashing code from the check engine light.

The dreaded check engine light comes on while you are driving only to signify that there is a problem. The specific error or failing part could be one of many different things that the computer senses for. You can bridge a set of wires in your car and send the cars computer into its error reporting mode so that you can find out whats wrong with your car.

Again, this instructable is only useful for people who have cars that report engine trouble via a flashing check engine light. This should be true for many cars made before 1996.

Here is a short video explaining the code deciphering process. For the full instructions check out the following steps.

Step 1 Find Your Vehicle's Diagnostic Plug

The first step is to find your vehicles diagnostic plug. The one in my car was a small blue plug that was bundled together with another factory plug beneath my glove box on the passenger's side of the car.

Just stick your head up under the dashboard on the passenger side and look around for a plug that isn't connected to anything.

If you don't have a Honda Accord then do a search online for where your plug is located and you will probably find some good leads.

Pull the diagnostic plug out from the plastic housing that is bundling it to the other set of wires under there (I think the second set of wires down there are sensors for the ABS brakes or the airbags).

Comments

[SPITFIRE226]

Auto Zone can ONLY do '96 and newer. I have a '92 Buick LaSabre and can not find anyone with a machine to diagnose the problem

[trebuchet03]

Just as a heads up... Cleaning the EGR may resolve your problem if it's just clogged ;) As opposed to replacing it outright. If I recall correctly, GM computers will give a code 11 or 12 (according to their code system - I'm not exactly sure which one :p) before giving diagnostic codes. That would be the "Yeah, I'm working" code. Not knowing that and trying to read codes makes a good head scratcher puzzle while you figure that out :p

[bridge7605]

Yes, code 12 on a GM vehicle verifies that the check engine light/computer diagnostic system is working properly. It will flash code 12 three times, then it will begin flashing each trouble code, three times starting with the lowest trouble code first, and continuing with each trouble code in memory, increasing numerically. When all trouble codes in memory have flashed three times, it will start over, (so that you know you're done reading codes) with code 12 again, and you can then remove the jumper. Oh, I guess I should have started out telling you that in order to pull codes on a GM vehicle, (with the exception of Cadillac) prior to 1996 you can use the same technique described in this instructable, but you'll be jumping A to B on the ALDL, (assembly-line diagnostic link). That plug is located near the driver's knees, on either side of the steering column, usually just underneath the lower edge of the dash panel. It will be a black plug, with 12 square holes in it, not all of the holes will have an electrical connector in them, but the A-B holes will always have connector visible in them. One more thing, don't forget to remove the paperclip, or jumper wire, because you'll be driving in diagnostic mode, which doesn't allow the vehicle to enter closed loop operation. Chrysler vehicles prior to 1996 are the easiest of all vehicles to pull diagnostic codes from. All you have to do on a Chrysler vehicle prior to 1996 is turn the ignition switch on, off, on, off, on, off, on and look at the check engine light which will be flashing trouble codes starting with the lowest one first and increasing to the largest one last. If anyone is interested in learning how to pull trouble codes from a Ford vehicle prior to 1996 let me know, and I'll write an Instructable for that. It's a little more complicated, and that is the only time you use an analog meter on a computer-controlled vehicle!!! Any testing on any computer-controlled vehicle which requires a meter has to be done with a HIGH IMPEDANCES DIGITAL MULTIMETER ONLY!!! The coil in an analog meter can, and will blow a number of circuits inside automotive computers. I hope this helps everyone, and have a great day.

[marr54]

I have a 1994 Lincoln towncar how to i get the trouble codes engine idles rough and stalls at red lights

[realclone]

"If anyone is interested in learning how to pull trouble codes from a Ford vehicle prior to 1996 let me know, and I'll write an Instructable for that. It's a little more complicated, and that is the only time you use an analog meter on a computer-controlled vehicle!!! Any testing on any computer-controlled vehicle which requires a meter has to be done with a HIGH IMPEDANCES DIGITAL MULTIMETER ONLY!!! The coil in an analog meter can, and will blow a number of circuits inside automotive computers. I hope this helps everyone, and have a great day." I am interested in the Ford procedure 1985 E-150 6cyl

[Tarek928]

Folks I'm sorry for digging up an old instructable, but I'm getting nothing when I jumper pin A to pin B. It's a 1988 chevy van G30 with 5.7 L TBI engine. I had fuel pouring out of the TBI area and so I replaced the top part with a rebuilt one, but now it's not starting. Should I conclude that the ECM is bad because I'm not getting anything in the dash when I jumper the 2 pins? Wires from ECM to injectors checked out ok. Thanks in advance

[frogs7]

Im interested in any information you have I have a 94 mustang 5.0 ho automatic it runs great but once in awhile turn it off and wont start turns over fine fuel presure none but when fuel presure is good still wont start untile you unhook the battery and it resets itself then runs great again untile next time . thnx email is [e-mail]

[avalerio1]

hey i have a mustang 1994 and the check engine light came on, i replaced the egr sensor. nd the valve.. the light went off for a few hours then it came back on, the next day it turned off in the morning.. then a few hours i was in the freeway and my car started to slow down i kept pressin the gas pedal and it did not go so i pulled to the side turned it off and tried turnin it back on and did not start .. the light came baack on and i tried usin the paper clip thing but it dont work,, any help??

[GEORGE45]

hi, my name is George, I'm trying to get the codes from y dads 1990 tempo, wenjt to autozone and they told me the car is to old for them to pull the code, any help would help, thanks. my email is [e-mail]

[acartists1]

Hello  Im very interested in knowing how to read codes on a 1992 Ford Ranger 4 cyc  thanks

[Toulouse]

hey, also it should be noted that Autozone can read your codes any time for free with a code reader, and they may be cleared by unplugging your battery for more than 10 minutes. neat stuff with the blinking light

[Adeyemi]

How can i get computer that can use to diagnostic automobile fault on cars ( fault reader).

[Derin]

replacement for a newer car of the same make/model e/x:Transporter 1992 to Transporter 2000

[Derin]

That may or may not work though.

[DragonDon]

All Chryslers have been built with an auto-code reader process. Check with http://www.allpar.com for specific years but basically pre-2000 was an on-off combo 3 times and a code was 'blinked' by either the check engine light of the MIL. Post 2000 displayed a 'p-code'. Details on the above site :)

[theonlycoolone]

thank you very much for writing this!!!!. I have a 95 accord and this tutorial made it a breeze to check for my "check engine" light. I got code 17 (VSS) is the sensor that the speedometer uses to calculate speed. which isnt life threatening either, but it is something I was already planning on replacing because it only works when it wants to. I have one question though, during the test, my D4 light was blinking (short, 4 times) is this a different code? or is this part of the test? I see on your picture that your d4 light is on, was yours blinking too? Thanks for the help in advance!

[Derin]

my uncle's speedometer also does the "I will only work when i want to" thing but driving on rugged terrain mostly fixes it since the wire shakes in place

[Derin]

my van is a 2006 VW Transporter...sniff...

[Derin]

but it has the check light combined with the reader

[dsandds2003]

I have found that some of those codes do NOT mean alot. They just point you to a possible problem in that circuit. Have had this happen MORE than once. Also different makes have different codes. Might be a good idea to pick up a chilton or similar manual. This is an excellent instructables by the way

[HAL 9000]

Neat-o, i have been having engine light trouble for some time now... But... I drive a 1991 Honda CR-X HF, My check engine light doesn't come on when i turn the car on. i first have to drive around for some period of time before it lights up, and the time period is very inconsistent. Usually it comes on as i accelerate through 50 mph, though not always the first time i hit 50 on a drive. it happens in 2nd, 3rd and 4th gears. sometimes it comes on as i hit 60, 65, 55, or 40 mph, or others, but these are the ones i have noticed. Usually 50 mph, like i said. So, i drove it out, got the light to come on, and stuck my wire in and nothing happened. i wiggled the wire for 10 minutes and jammed it in and wiggled more and nothing happened. i turned the engine on and off (re-setting the light) and drove with the wire in it untill it came on. nothing happened. so basically i fiddled with this trick and tried every combination of doing this and that to get the damn light to blink to no avail. If anyone has any idea what this could be caused by PLEASE let me hear your idea! i LOVE this car, and i love the 45+ mpg, and this light problem has me worried a bit. im going to have it serviced soon, but as others have said here many places dont use whichever gadget will check the engine light, and i would like to have some idea as to what it is when i go to the mechanic, so i can walk in and say "oh yeah, the check engine light comes on every now and again because..." and this is the part i wish i knew Anyway, nice instructable, good tips, wish it worked on my dear little car.

[noahw]

The codes can only be read when the car is stopped and the engine is off. The car computer flashes the codes when the key is in the ignition and turned to position one. So, I don't know how driving around and then shoving the wire into the plug will help out too much. My light doesn't come on when the car starts either - it takes about 5 minutes of driving before it lights up. The computer remembers codes however. So, even if the light hasn't been coming on, you can still read old codes from the computer by using this method. In order to resent the codes you need to disconnect the battery from the car. Once it's disconnected, it will forget the codes and you can start the whole process over clean. Try disconnecting the battery, driving around for a day, then parking the car and turning off the ignition. Then, put the wire in the plug and turn the key into position, but don't start the car. It should work then. Hope this helps, Noah

[incorrigible packrat]

My advice is to get one of those repair manuals (Hayes, etc.) for your particular car. It should show the location of the diagnostic port, which pins to jump, list of trouble codes, and all sorts of other useful information. I learned this lesson the hard way, when my old 92 Mazda started overheating. I took it to a Mazda dealership, with the idea that they could do a diagnostic on it. TWELVE HUNDRED DOLLARS later they: Told me they "no longer had the machine to read OBD 1 codes". Monkeyed with the wiring, so that the two-speed cooling fan no longer worked properly. Insisted on replacing suspension parts before allowing the vehicle to leave the shop. Failed to replace the engine coolant, which was the problem in the first place. (It's supposed to be green, not black) They probably did other stupid things which I have forgotten, as well as giving me a whole lot of 'tude and not even offering a loaner. btw the dealership in question is Bank Street Mazda, in Ottawa, Canada. Never, ever do business with them. They are crooks. Don't buy new Mazdas either, they probably have sucky Ford engines and other parts hiding in them.

[Flea]

You can bring your car to Advanced Auto Parts or Pepboys and they'll hook up your car to their handheld meter and tell you what the error is for free. There's no need to go through the hassle of running wires to it. You might have to tell them where the diagnostic connector is.

[noahw]

It's true, they do diagnostics for free, but that is for new cars that require a hand held unit. This method is for reading older check engine lights that can't be hooked up to code readers.