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What are Diagnostic Trouble Codes?

By Matt Brady
Updated May 23, 2024
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Diagnostic trouble codes, known as DTCs, are alphanumeric codes that a vehicle's computer outputs when it detects a malfunction. These codes are transmitted by a vehicle's on-board diagnostics (OBD) system and can be accessed using a diagnostic scanner that plugs into the OBD connector. Every vehicle made after 1996 comes equipped with an OBD-II computer—the modern OBD system.

Vehicles today are capable of relaying thousands of diagnostic trouble codes to anyone with the equipment to read them. These codes are primarily referenced by auto services. The savvy driver, however, can also reference a car's codes to diagnose a problem and, in the process, possibly avoid an unnecessary trip to the car shop.

OBD technology was initially developed in the late 1970s in an attempt to better regulate car engine emissions. Those early computers, however, were fairly limited in what they could detect. Jump forward 20 years, and OBD computers had taken a mighty leap, with computers able to read and diagnose a far greater number of car functions. By 1996, with the advent of the OBD-II computer, the modern system of diagnostic trouble codes was born. Since then, every car has been equipped with OBD-II connector ports, sometimes referred to as Diagnostics Link Connectors (DLC) or J1962 connectors.

OBD-II systems are capable of outputting thousands of codes. Codes can either be read by a personal diagnostic scanner or by a certified technician at a car shop. If using a personal diagnostic scanner, one can consult a trouble code manual or any one of multiple websites that provide a thorough listing of trouble codes and their translations. Without any sort of reference, anyone who isn't a certified technician would find it difficult to accurately decipher trouble codes.

Diagnostic trouble codes don't only communicate with individuals, they also communicate with the vehicle. For example, certain codes tell a vehicle when to turn on the "check engine" light. Sometimes, the "check engine" light is triggered by a trouble code that indicates serious car troubles. In other cases, the trouble code that caused a "check engine" light might merely be indicating that a fuse needs to be replaced. Thus, having a personal diagnostic scanner can go a long way in helping an individual car owner figure out whether a "check engine" light actually necessitates a trip to the car shop. A diagnostic scanner can also give a car owner the option of turning off the "check engine" light after assessing the diagnostic trouble code.

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Discussion Comments
By anon287423 — On Aug 25, 2012

I had the o2 sensor replaced on my 06 Chevy Aveo and the catalytic converter replaced and the light still came on. I was told that the intake manifold was the culprit because it had a pinhole leak. I had it replaced and now I'm back to the same place. The light's on again and it's popping up the same code. Now what?

By cardsfan27 — On Nov 14, 2011

@JimmyT - OBD scanners aren't really that expensive at all if you want to check your own codes. You can find them several places online for around 20 dollars. I have a couple of friends who like to work on cars, and they have one that they share between them. I have had them use it to read my engine codes before. The one they have is even Bluetooth enabled, so there are no wires or anything.

Once you get the code, you can just search for "OBD diagnostic troupe codes" online, and you will find plenty of places that will tell you what the problem is.

Like someone else mentioned, though, if you don't want to buy one, you can always go to whatever auto parts store is closest to you.

By JimmyT — On Nov 14, 2011

So, it is possible to buy your own OBD scanner to read the diagnostic trouble codes for a car, right? Does anyone know how much they normally cost and whether or not it is even worth it to have your own? Also, do they make different readers for different makes of cars, or will one reader work for every car?

Once you actually use the scanner and come up with whatever engine code you have, is there just a website that says what all the codes are?

I have an older car and at least once or twice a year the check engine light will come on. I always have to end up taking it to the mechanic. Most of the time it is something fairly simple that I think one of my buddies could fix. Only a couple times has it been anything really serious.

Besides engine problems, can the scanners pick up on other things? I know a lot of newer cars measure things like tire pressure and fluid levels. Would a scanner be able to read into those things, too?

By Izzy78 — On Nov 13, 2011

@nony - You brought up something important in that, you can go to any of the major auto parts stores like Autozone or Pep Boys, and they will check the OBD codes of your car for free and tell you what it means. Obviously, it is with the idea that you'll buy whatever parts you need from them, but there is nothing saying that you can't go somewhere else later on.

I have done that on a few occasions to see what my check engine light codes were. Most of the time you will find it is something minor that won't make much to fix.

Just another helpful hint, if you have a check engine light come on, try to tighten the gas cap. If the light doesn't go off immediately, turn the car off and back on or drive around for a little bit to see if it goes off. If it is still on, let the car sit overnight and start it back up the next day.

If the light went off, it was most likely that some air had gotten into the gas tank and was messing with the oxygen sensor. This isn't anything that needs to be taken to a mechanic. If the light is still on, it is probably something else.

By everetra — On Nov 13, 2011

@Charred - Regardless of what the codes actually mean I think it's good practice never to drive around with the check engine light on for too long.

Some people find out that the codes refer to something innocuous, and don't bother to fix the problem. So that check engine light remains on.

That's okay for that problem, but what if something else happens that's more serious? Since your check engine light was already on to begin with, you won't know about the more serious problem. You might wind up with a major repair bill.

So the bottom line is, fix whatever is causing the check engine codes to come on, whether you think it's serious or not.

By Charred — On Nov 12, 2011

@nony - That’s a great point. Another important point is to know how old your car is because that will affect what type of OBD technology is used.

If it’s prior to 1996, then it uses the original OBD technology and those connectors will be located somewhere else inside the compartment of your car. They won’t be as easily accessible, although any qualified mechanic will be able to locate them.

With OBD II however you can just buy a code reader yourself and plug it into a slot to read the OBD fault codes yourself.

By nony — On Nov 12, 2011

You should understand that after your check engine light comes on, your OBDC diagnostic trouble codes simply point you to the general area where the problem may be occurring.

They may not pinpoint the actual problem itself, but they can tell a diagnostic technician where to look. So don’t read a code and immediately go off and buy a part, thinking you know for sure how to fix it.

I say this because last year I had the “check engine” light in my Honda come on, and I took the car to AutoZone to run a free check on the codes. The result came back, “P0420 OBD-II Trouble Code: Catalyst System Efficiency Below Threshold (Bank 1).”

Well, if you check up on that code, it could be the catalytic converter or the oxygen sensor, as the most likely culprits. I almost bought an oxygen sensor, hoping that was the more likely (and cheaper) solution. But I held off.

I had a mechanic look at it, and sure enough the catalytic converter was bad. I got a used catalytic converter that was still working from another mechanic, and had it installed. It was a lot cheaper than a new unit, although still more expensive than an oxygen sensor.

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