For years you were told to stick to the Strategy Based Diagnostics approach to diagnosing cars, and although that is still an organized way to tackle tough problems, the game is changing. Introducing, "Functional Diagnostics", the 21st century's update to your already established diagnostic routine.

A little background;

With the ever changing complexity of today's vehicles and the proliferation of  serial data busses and control modules, (see the article on GMLAN) the idea of taking on a tough problem was growing more involved than the usual "Strategy Based Diagnostics" could handle. When a vehicle has to perform a certain "Function" and that "Function" can be controlled by many modules, tah dah - Functional Diagnostics!

How it Works

  On a vehicle with the new LAN system(s), there will be a new option on the Tech 2 where you can perform an operation similar to the Class 2 Diagnostic System Check. To navigate to the check, first fire up the Tech 2 and "build" the vehicle. Once the necessary info is in, the screen for the next choices will look a lot like what you are used to seeing in eSI. This has been built into the software to make your interface between Scan Tool and Service Information more uniform. The illustration to the right is for a 2004 Cadillac XLR, it will be similar on other models as well. Editor's note: Not all vehicles with the LAN system will support the Vehicle Control Systems option, the best way to know-if it is NOT an option, it is NOT supported.

 Once the test begins (total of approximately 2 and a half minutes) the Tech2 will "poll" all of the modules, regardless of the data bus they are on, to see if they are awake and responding. This test differs from the Class 2 Diagnostic Circuit Check in that the Tech 2 is not just a passive observer looking at State of Health messages and then creating a list from that. Instead, it actually goes out looking for certain modules that are supposed to be on the vehicle. Once again though, check the SPID label to make sure what options are installed. As you can see from the screen to the right, while it goes through the check it waits for an answer from the modules. Once it gets the answer it will look like the following. (Editor's Note: The pins that the module communicates on in the DLC will be displayed at the bottom of the screen and will indicate which data bus is on. In this case the HVAC module is on Pin:2, which indicates a Class 2 module)

DTC Symptom Numbers

The DTCs for this vehicle include DTC symptoms. A DTC symptom is a 2-digit number which adds additional detail to a DTC. The DTC symptom provides the technician additional information without requiring a large increase in the number of new DTCs.

The DTC symptom is made up of 2 alphanumeric digits. The first digit following the DTC indicates the DTC symptom category. There are 16 possible categories available in the range of 0 through the letter F. Currently there are 8 categories in use, 0 through 7. Some of the  categories together with their definitions are given below.

The second digit of the DTC symptom is the subtype of the DTC symptom. Some of the  subtypes and their categories, together with their definitions, are given in the following table. DTC symptom 00 is a special case. If 00 is displayed, only the base code number and its description apply. Information regarding the fault will be provided in the code setting criteria.

DTC Symptom Number Example

The DTC symptoms associated with each DTC provide more information about the fault that caused that DTC. An example of a DTC displayed can be B1451 05 where the B1451 is the DTC, and 05 after the space represents the DTC symptom. While the DTC indicates that the fault is in the accessory power circuit, this DTC symptom indicates the circuit is shorted to battery or open. Another possible symptom for this code is B1451 02, where B1451 indicates the accessory power circuit, and 02 indicates the circuit is shorted to ground. This may also help in solving intermittent problems as well. If a vehicle comes in with a history DTC and you are unable to duplicate it, with a symptom number you have a better idea of what type of failure it was. 

An important note is that the Symptom Number system does not apply to Powertrain codes as they are governed by the rules of OBD-II and must adhere to those regulations

When Contacting Tech Assist, with a problem that includes a symptom number, make sure to include it in your information that you give to the TAC Consultant, it would help them greatly in diagnosing your problem.

 Mik Stubing - Lead Instructor GMTCNY