After 10-plus years of dealing with OBD II, it should be apparent that we are on a learning curve that probably has no end. Almost daily we have a VAG model come into the shop with a check engine light on, or a "state OBD II failure" form. Although we have the latest scanner software packages, some of these failures can still be elusive to resolve.

In this article, I'll revisit some of the more common failures that we continue to see and how we resolved them, saving you time and cost for your customer. As always, throwing parts at a problem is not the best tool for OBD repairs. Knowing how a component is designed to work is always the first step in locating a failure.

Whatever the reason for the MIL to be on, there was a sequence of events that lead the ECM to conclude there was a problem. A visual inspection should always be the first step in any diagnostic procedure. After pulling codes in the case of an OBD failure, a look at the system involved just might turn up an obvious fault without any test equipment.

The most important element of VW OBD II repair is having the right equipment. You need to not only pull codes, but be able to track data and perform at least basic activations to test systems' operation. To make repairs, you will need to locate the cause before replacing parts. As the parameters for components get narrower, the need for more accurate data becomes paramount. When choosing from among the...