Decoding OBD-II Diagnostic Trouble Codes

The Diagnostic Trouble Code system is a huge help to the home mechanic. That's because the car will be able to tell you what’s wrong through the OBD-II system. If the car's computer ever encounters a problem, a fault code or Diagnostic Trouble Code (DTC), will be stored within the car’s ECU memory. With a scanner plugged into the car’s On-Board Diagnostics (OBD) port you can read the list of codes stored, and therefore find out what the problem is.

The codes follow a formula making it easy to generally know what the issue is before even looking at the chart below.

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 = Standardized (SAE) fault codes

1 = Manufacturer specific 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 = Standardized fault

3 = Ignition systems or misfire

03 – misfire on cylinder 3

OBD II trouble code chart*

* Not all codes apply to all models