OBD Communication Problems:


Q: Inspection stations may encounter vehicles that have problems communicating with the OBD inspection machine. What should stations do to resolve these communication issues?


OBD communication issues can be caused by various issues.  Here are several common problems to check for.


1.      Connector wiring issues

Every 1996 + gas powered vehicle, and 1997 + diesel powered vehicle under 8501 lbs. certified to be sold in the United States is OBD certified and uses a standardized connector (DLC) and connector wiring. Different communication protocols are used, but all protocols use the same pins to supply power and ground to the emissions machine. Pin 16 is the standard B+ power pin. Pin 4 and 5 supply ground for chassis and signal ground. Check for 12+ volts between pin 16 and each of these ground pins.


The rest of the standard-designated pins are digital signal pins of various protocol. Any non-designated pins may be used for manufacturer-specific use but are not used for generic OBD communication.

** When checking the wiring at the DLC, it is easy to damage the terminal pins. Do not spread the terminal contacts by inserting test equipment that is too big, such as paper clips or voltmeter leads. Only insert test leads that are the correct size.




2.      Communication interference

One possible cause of communication problems with off-board OBD devices such as I/M inspection machines is a faulty module or network communication node sharing the OBD network on the vehicle.


Generic OBD devices such as the I/M inspection machine communicate with the necessary OBD module(s) through a communication network using one of several communication protocols. In general, several modules are connected to the OBD connector and communicate sharing the same wire or wire pair. Any one single module communication fault has the potential to affect the signal on the entire network and disturb communication throughout all of the systems.

Some off-board devices are more sensitive to this interference than others. This type of fault may be found by checking secondary modules on the network for communication problems or trouble codes using an OEM scan tool. Some aftermarket scantools supporting manufacturer-specific modes may also have this capability in varying degrees.

On most vehicles, if you suspect a certain module is the problem, you can disconnect the module from the network by unplugging the module and seeing if the problem is resolved. However, on some vehicles, network configuration or other issues caused by disconnecting certain modules will make this diagnostic much more difficult or impossible in some cases.

For a large amount of trouble vehicles, the problem will be resolved by unplugging the ABS module.

The following makes are the most common with trouble ABS modules:

·         AUDI

·         Volkswagen

·         Mitsubishi

·         Porsche

·         MAZDA

·         Subaru

Other vehicles may also have issues with the ABS module.  On most vehicles, it is a fairly quick and easy diagnostic step to unplug the ABS module and see if the problem is resolved.

Other known module issues include:

·         Select 2002 Ford Explorers have a communication issue caused by the instrument cluster module.   If you encounter this issue, unplug fuse 2.21 5A (instrument cluster) located in the central junction box under the dash and attempt the OBD inspection while the fuse is unplugged.

·         Select 1997-1998 Porsche Boxster vehicles have a communication issue that may be caused by the Transmission Control Module. Unplug the “Tiptronic” TCM, located in the trunk and attempt the OBD test.


3.      Aftermarket Accessories

Certain aftermarket installs can be tied to the OBD network and cause inadvertent OBD communication problems. Remote start modules, aftermarket remote lock modules, Vehicle GPS tracking modules, alarms, etc. can be tied into the OBD system under the dash, under the hood, near the fuse box, or potentially anywhere the wiring harness runs throughout the vehicle. These devices most often affect GM product vehicles, but will potentially affect any make and model. Look under the dash, under the hood, near the fuse box, and all along the wiring harness to locate any aftermarket wiring harness or module. If any of these types of aftermarket accessories are found, uninstallation of device and repair of any related wiring is recommended to assure proper function of OBD and other systems. If an aftermarket radio is installed, it may cause OBD issues on some vehicles if the wiring harness was not modified correctly. To diagnose issues related to aftermarket installs, disconnect device, ensure wiring is correct and not damaged, and try OBD test while disconnected.


4.      OBD-Related Emissions Recalls

From time to time certain vehicles need to be recalled to fix OBD communication issues. To check for these recalls, check the NHTSA OEM Database and input the vehicle information. If a recall has been issued, a document will be stored in that database for reference.