A diagnostic trouble code (DTC) Indicates the enable criteria for a monitor test has failed outside of a minimum or maximum test limit. If the MIL is illuminated tailpipe emissions have exceeded 1.5 times the Federal Test Procedure (FTP) standards. This can occur on the first or second consecutive trip.

All DTCs represented by a five-digit alphanumeric code. The first letter indicates the function of the monitored component or system that has failed:
P = Powertrain
B = Body
C = Chassis
U = Indicates a network or data link code
The second digit is represented by a number that indicates who is responsible the code:
0 = Society of Automotive Engineers (SAE)
1 = Manufacturer specific

The third digit indicates the specific system in question. Numbers one through seven indicate a powertrain related problem. The number eight is reserved for non-powertrain related problems:
0 = Total system
1 = Air/Fuel metering control
2 = Air/Fuel metering control for injector circuit malfunctions only.
3 = Ignition system or misfire
4 = Emissions Control
5 = Vehicle speed Control and Idle Control system
6 = ECM and Computer Input/Output Circuits
7 = Transmission
8 = Non-Powertrain related
The fourth and fifth digits represents the component, system, or area experiencing the problem.
Using DTC P0306 shown in figure 12-17 is described as:
P = Powertrain related
O = Manufacturer defined DTC
3 = Ignition system or misfire related problem
06 = Cylinder #6 misfire detected.This MODE retrieves from the PCM all stored emission related DTCs.