OBD II ...Simply Explained
Technical article by:
Bruce Bonebrake
Managing Editor at BATAUTO.com
"What is OBD II and what about OBD I?"

    Since the late '70's early '80's vehicles have been equipped with sophisticated electronics to control vehicle emissions and performance. Through the years several systems have been used, and keeping up with the differences from year to year was a real chore, to say the least. Multiple scanners that attached to the vehicles on-board computer systems were needed, and it was not uncommon for the scanner to become obsolete from model year to model year. These were the days of the "pre-OBD I "(On-Board Diagnosis-First design) systems. Beginning in '88 CARB (California's Air Resources Board) and the EPA (Environmental Protection Agency) required vehicle manufactures to include a "self diagnostic" program capable of identifying an emission related fault in the On-Board Computer systems. The first generation of On-Board Diagnostics came to be known as OBD I. The CARB conducted studies on the OBD I vehicles and found that the system was not capable of detecting an emission related component unless it had failed. The components that had not failed completely were unable to set a DTC.

Also, it was found that some systems not being monitored had failed and yet the vehicle would pass an Emissions Test as the failed part would not be a factor unless the vehicle was being driven or under a load. The CARB and the EPA passed new laws that would address the problems found during the OBD I case study. These new laws and requirements are known as OBD II. Since mid-'94-'95 some vehicles were equipped with the second design, OBD II systems. In '94-'95 only select models were equipped with this new system, and in '96 every vehicle sold in the United States were equipped with the OBD II system. You might be asking, ok, so what? Well, this was the single and largest improvement made to diagnosis and repair of the On-Board Computer systems since their introduction. The DLC (Data Link Connector) that a scanner attaches to, is virtually the same for every vehicle, and the "Generic" DTC's (Diagnostic Trouble Code) are the same for every vehicle. The terminology was changed to terms that would be used by all manufactures. Before this, the computer (PCM), for example, could have been called a Processor, ECU, Control Module, ECM, etc. To know what the name of a part on a specific system was called or how it functioned, or was located, was tough when you worked on multiple vehicle model.

Do you remember the early home computers? The box that connected to your TV set and allowed you to play games and do light bookkeeping? Then the 286, 386, 486 computers each being a little faster and better than there predecessor? Look at the OBD II system on your vehicle the same way, faster and "smarter" than any other system used in computer equipped vehicles. This is good news for everyone. The vehicles perform better (fuel economy, performance, longevity) and the "self-diagnostics" are simply amazing. If 20 years ago someone would have told me that the on-board computer would be able to tell me that the fuel cap was loose or that a specific cylinder was misfiring I would have laughed. Well, that is the age we are in now folks!!

"Does my vehicle have OBD II?"
  To determine if your vehicle is equipped with the OBD II system is fairly simple. The DLC (Data Link Connector) will be located under or around the drivers side of the dash. The connector will look exactly like the illustration below:

Keep in mind that if your vehicle is '96 and newer, Federal Law requires that ALL vehicles (cars and light trucks) sold in the United States MUST BE OBD II Compliant. This includes ALL Domestic, Asian and European vehicles.

(OBD II Connector 16Pin located under the dash..notice the unique shape, unlike the rectangular style commonly found on GM OBD I vehicles. The DLC is usually located 12" from the center of the instrument panel, on the drivers side of most vehicles. It should be easily visible from a kneeling position outside the vehicle with the door open.)

You looked and found the DLC connector and it looks exactly like the illustration so the vehicle is OBD II right? Well, maybe.. if your vehicle is pre-'96 there is a chance that the connector is the OBD II style and yet the system is still an OBD I. Ok, so how can I find out for sure if I have an OBD II system? Again, a simple task..if you look under the hood in the engine compartment you should be able to find the Vehicle Emission Control Information (VECI) Label. Depending on the application, the VECI Label may be located on the bottom side of the hood, the radiator fan shroud, the core support or like on this '01 Olds Alero, on the strut tower:

(NOTE: Some Asian and European vehicles DLC is located behind the ashtray and the ashtray must be removed to access the connector)

The VECI Label will state whether the vehicle is OBD II compliant and will state "OBD II Certified". If your VECI Label is not present, I would strongly suggest contacting the vehicle dealer near you and ordering the replacement  Label. There is vital information that may be needed when performing engine performance repairs located on the VECI label.

"Now that I am positive I have an OBD II system in my vehicle, how do I know it is functioning correctly?"
  When a problem within the system that will effect the vehicle's emission output is noted, the MIL (Check Engine) will illuminate to alert the driver that a problem exists within the system. Also, the system will set a DTC (Diagnostic Trouble Code) that can be retrieved using an OBD II Scanner or Code Reader. The DTC will lead to the direction in which the fault occurred. This is one area that is misunderstood. The DTC is a "helpful tool" that will be key in determining what happened within the system. One thing to keep in mind, the DTC is a STARTING point in most cases. For example, a PO301 would be a misfire was detected on #1 cylinder. The first thing that comes to mind is the spark plug, and there is a good chance the spark plug could be failing, but, a defective fuel injector, spark plug wire or COP (Coil Over Plug) unit is failing and cause the DTC to set. Then all of the other possibilities, basically everything that would have to do with the cylinder performing is a possible reason for the DTC to set.

"What is the difference in a Generic and Enhanced DTC? Does my vehicle have one or the other or both? What are they?"
Every OBD II vehicle has to comply to strict emission standards. When the vehicle is new, this is easily achieved, but what about after 50,000 or even 100,000 miles? Every OBD II vehicle is required to able to determine if a fault within the system that would cause excessive emissions to be expelled from the tailpipe in the form of a DTC. The Generic DTC's are a list of mandatory trouble codes that must be present and able to be displayed should a fault arise. There are many Generic DTC's and not all of them will be used on every vehicle. The ones used depends on the yr/make/model/engine of the vehicle. The Generic DTC's are also designed to be able to be retrieved using a standard OBD II Scanner or code reader. The Enhanced DTC's are DTC's that are vehicle specific. Simply put these are DTC's that have been added by the manufacture to further "Enhance" the diagnosis capabilities of the vehicle. Without this option every vehicle would be a clone to one another. A simple example of an Enhanced DTC would be:

Ford Vehicles..
                        P1227 Wastegate Failed Closed

  Here is a chart that is an example of a misfire DTC:
  For further information on OBD II scanners and code readers, please check out our product reviews at:

BAT Auto Resource Center

Thanks for taking the time to read my article!! Your feedback is greatly appreciated!!
You can email us at:

Bruce Bonebrake