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Uncoding OBDII Diagnostic Trouble Codes

What are… Diagnostic Trouble Codes

Modern cars are very clever, and most of them will actually be able to tell you what’s wrong if they have a problem. If the car ever encounters a problem a fault codes, or Diagnostic Trouble Codes (DTC) to give their full name, will be triggered and stored within the car’s ECU. You can then gain access to this via the car’s On-Board Diagnostics (OBD) port to read the list of codes stored, and therefore find out what the problem is.

Each of the DTCs will be unique – that’s how you can pinpoint the exact fault – and to list them all would take up more space than have available, and would make for a very boring read!

But, thankfully, most codes follow a similar format, and understanding how they break down will give us a very good indication of what area the problem relates to.

The First Letter…

The first letter denotes what part of the car is at fault.

P = Powertrain

B = Body

C = Chassis

U = Network

The First Number…

The first of the four numbers will be either a ‘0’ or a ‘1’.

0 = Standardised (SAE) fault codes

1 = Manufacturer specific codes

What are… Diagnostic Trouble Codes

The Second Number…

The second digit denotes which of the car systems is at fault. There are eight different categories;

0 = Fuel and Air Metering and Auxiliary Emission Controls

1 = Fuel and Air Metering

2 = Fuel and Air Metering (injector circuit)

3 = Ignition systems or misfires

4 = Auxiliary emission controls

5 = Vehicle speed control & idle control systems

6 = Computer & output circuit

7 = Transmission

The Third And Fourth Numbers…

The third and fourth digits are simply used to define the exact fault code in question.

In our example we can see that the DTC is P0303.

P = Powertrain fault

0 = Standardised fault

3 = Ignition systems or misfire

03 – misfire on cylinder 3