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