OBDII stands for second generation On Board Diagnostics. Shortly after computers were being put into cars, General Motors gave theirs the ability to "communicate" to a technician and store codes when they recognized a problem with the system. Other makes did the same over time. These early self-diagnostic systems were called OBDI.

With OBDI, the system used from the early 80's through 1995, as soon as the computer saw a problem it would turn on the light, but if you turned off the key and restarted the car the light would stay off until the computer saw the problem again. This was a passive system that didn't do any self-testing.

OBDII became mandatory in 1996 and with some of the OBDII codes, the computer has to see the problem on at least two or three different trips in a row before it turns on the light. Once the light is on and the problem goes away or is fixed it may take two or three separate trips from cold to warm to turn the light off again. If you do a lot of short trip driving it could take a lot longer. This makes it harder to tell how often an intermittent problem may be happening, but with OBDII the computer monitors a lot more things and has much better diagnostic abilities and is continually performing self-tests to find problems that could cause mileage or emission issues.

Diagnostic codes merely point to the system the computer sees the problem. Replacing parts without testing is a gamble and can be costly.

OBDII also requires the computer to test its emission systems regularly which was not required in OBDI. This requires a lot more sensors and components for self-testing. Systems it conducts self tests on are: EGR operation, closed fuel system vapor leaks, catalytic converter efficiency, engine misfire, transmission slippage and various others. If any of these tests fail the computer will set a "trouble code" and eventually turn on the "check engine light". If the engine is misfiring it will flash the check engine light because of potential catalytic converter destruction.

When the light comes on it could be for a simple emission problem or a more serous engine or transmission control problem. Most shops will pull OBDII codes for you at no charge. If the problem is a simple one and not fixed, when a more serious problem arises you won't know, because there is only one light.

Also if a sensor quits working or is supplying bad information the computer will often  compensate by using another sensor to calculate the information. If that sensor fails also, you may be left waiting for a tow.

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Wayne's Garage
27 E 27th in Eugene, Oregon
333 Q Street in Springfield, Oregon