DTC Comparison
(Why a generic OBD-II scan tool can be useless)
Posted by Andy Smith

Here's a real-world example from my car from September 2002.  My car (a 2000 GTI 1.8T) had stalled out several times over a period of a few weeks.    One evening leaving work, the car stalled, but I was able to restart it. The Check Engine Light  was not on, but I scanned for codes. Here's what a generic OBD-II code reader reported:

No fault codes, according to OBD-II, everything is fine !?!?

Here's a screen-shot from a second OBD-II program:

Same results, no fault codes of any kind  !?!?

Now let's see what VAG-COM has to say:

VAG-COM reports  a total of 10 Fault Codes  !!!!!

Here's more of them:

Now using the troubleshooting sections and wiring diagrams  in my Bentley CD,  I tried to figure out what all of these components  that are supposedly "Short to Ground" have in common..?  
Without too much trouble, I found they all get their power  from the fuel pump relay!  To test the theory, I cleared the codes.    Then yanked the relay with the engine running at idle.   The car stalled instantly, and sure enough,  the exact same  codes reappeared.   Went to the dealer and $22 later my car is running perfectly with a new fuel pump relay!

Needless to say, if the only tool I had was a generic OBD-II scanner, my car would still be stalling out intermittently!

Click Here for more information about the differences
between VAG-COM and generic OBD-II scanners.