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  • Writer's pictureErick

A Rookie’s Guide to Understanding Vehicle Error Codes:

Today we will demystify those lights that send shivers down the spine of every car owner, yes that little light of yours that you have always let shine:

What do they necessarily mean?

How can you decode them once your diagnosis machine present the codes to you?

Worry not, transcribing the codes is not as difficult as many car owners have always thought it is, by the time we are done with this piece you will be an informed rookie and a resourceful DIY guy to yourself, foes and friends. DTC (Diagnostic Trouble Code) means the system has detected a problem in one of the onboard systems and is spitting out a code that relates to the specific section that has triggered the fault. Faults can come from any electronic system or sensor in your car so there are thousands of possibilities and all of them are converted into a simple 5 character fault code, (e.g. P0399 , P0405, U1000 etc. List can be long as the SGR)

….“I hope we are together up to this point…

The 5 digit code is broken down into 4 sections, as outlined below.


The first digit takes a letter form as below

B = Body codes (They relate to components that provide safety, driving assistance, comfort, convenience etc

C = Chassis Codes ( They relate to Mechanical systems such as ABS, Steering, suspension etc)

P = Powertrain (engine, transmission etc) codes

U = undefined codes: Network and Vehicle integration, communications error etc

….. See it wasn’t algorithms and numbers don’t lie kind of fallacy like you feared it would be…. We aint done yet, infact we aint started yet.

Remember when I enrolled you to this crusade a few paras back I mentioned that an error code has 5 digits, the first digit is what I have just explained. Now we head right into the second digit.


The second digit takes the form of a number

0 = Generic fault codes common in most vehicles that are on the OBD-II platform (modern cars, post 1996 cars) hence common across all manufacturers

1= Brand specific fault code or manufacturer specific code. They are unique to specific makes and car brands.. ( you can have the codes having a 2 or 3, but for today I want to make it simple, I will avoid this area)


The third digit takes form of a number as listed below (it refines the 5 code further so that we get to know which location the error is for, shows the subsystem the error is from)

1 = Fuel and Air Metering

2 = Fuel and Air Metering Injector Circuit

3 = Ignition System (Including Misfires)

4 = Auxiliary Emissions Controls

5 = Vehicle Speed Controls and Idle Control System

6 = Computer Output Circuit

7 = Transmission

8 = Transmission

4th & 5th DIGITS

The last 2 digits are combined into one. They range from 0 to 99. They Indicates the specific area of the subsystem that triggered the fault. In short, they narrow down and tell you for example, that it’s a misfire and where it is happening (like misfire in cylinder 2)


Previously, the second digit defined the sub-system of the codes. However, as of 2016 things were tweaked a little bit. With the increase in DTC usage and the introduction of new technology to vehicle systems, it was necessary to remove the grouping of DTCs into functional areas.

The last two or nowadays three digits define the actual fault description. These numbers will tell the particular problem and each code is defined separately. There’s no formula to decode these codes automatically. But worry not, modern diagnostic machines will do the hard lifting for you. You only need to connect your comprehensive diagnostic machine and it will translate all these jargon for you and even go further as to give probable area on which to start your troubleshooting.

Don’t put the cart before the horse

Always investigate as the code is not always so direct, for example if you have a fault code for the O2 sensor it does not always mean the sensor is faulty, it could be the wiring or the connection so be sure you test any part first. Often with sensors there is an acceptable resistance of the sensor, so grab the car/manufacturer manual and find the basic resistance test for the sensor, then check if it is matching to your sensor. If it is, inspect the wiring all the way back to the ECU for continuity. This step is key, unless you test the circuit properly you could end up buying parts you don’t need.

@DiY254 we continue to empower you by bringing you the latest, well researched and engineered diagnostic machines to meet the daily needs of the 21st Century Automobile - Call us on 0722 73 73 73 to inquire about any Autel or Launch Diagnosis Machine

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