$03 Emission Related Diagnostic Trouble Codes (DTC)
A diagnostic trouble code (DTC) indicates that 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. MODE $03 reports a DTC once the MIL is commanded ON. This can occur on the first or second consecutive trip. MODE $03 reports a DTC once the MIL is commanded ON. A DTC with the MIL ON is called a confirmed or current DTC. If the MIL is OFF and a DTC is still present, this is referred to as a history DTC. A history DTC remains in memory for 40 warmup cycles if the same code does not reoccur. It can also be cleared using a scan tool or by removing power from the ECM for a while. The length of time depends on the manufacturer.
A five-digit alphanumeric code represents all DTCs. 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 for 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/Idle Control system
6 = ECM and Computer Input/Output Circuits
7 = Transmission
8 = non-Powertrain related
The fourth and fifth digits represent the component, system, or area experiencing the problem.
DTC P0306 is described as:
P = Powertrain related
0 = Manufacturer-defined DTC
3 = Ignition system or misfire related problem
06 = Cylinder #6 misfire detected. This MODE retrieves all stored emission-related DTCs from the PCM.
Types of Diagnostic Trouble Codes (DTC)
The ECM can set two kinds of DTC.
1. Functionality DTCs are related to electrical problems set by an open and short circuit. The image below demonstrates a functionality DTC.
2. Rationality/Normality/Plausibility DTCs are performance-related problems. They are set by comparing similar inputs/outputs to see if they make sense. The customer's concern is loss of power under load. The image below shows three related PIDS: TPS, MAP, and RPM. In this case, the ECM must determine which of the three does not make sense. Can you identify the outlier PID that the ECM might flag as incorrect?
Live by codes, die by codes. Sometimes, the ECM can be fooled because the reporting component is malfunctioning. The BARO and FTP PIDs must be checked KOEO and KOER to start an EVAP diagnosis. A DTC for an out-of-range sensor will not be set if the BARO or FTP reports in the yellow zone. This can lead to a false DTC, like a Large Leak because the FTP is not reporting the correct EVAP system pressure. If the BARO reports in this range, it could cause a driveability concern or the suspension of non-continuous readiness monitor tests.
