[s-cars] check engine light: officially bewildered

Mike Fitton rfitton at vt.edu
Tue Sep 15 19:49:44 PDT 2009

Lemme start at the beginning.  Some may recall that I thought it would 
be a good idea to hardwire a PC into my trunk.  So I did.  And then I 
got one of those Chinese vag-com cables off ebay to do that sort of 
thing.  I had it permanently plugged in and under the carpet.  Worked 
great.  I could pull codes and scope sensors whenever I wanted.  All was 

At some point I needed to rearrange some of the wiring in the car.  Part 
of that wound up seeing the cable unplugged from the computer for 
several months.  And at some point my check engine light started coming 
on.  Not sure on the correlation with that.  I left the USB side of the 
cable in the glovebox so I could use it.  I kept plugging my laptop in 
and wouldn't get any codes, yet the CEL remained.  I know that the light 
without a code is usually a clear indicator of impending CPS doom, so I 
replaced it.  Engine always starts as if the CPS is fine, before and 
after replacement.  CEL remains.  No big deal, so I have a new 
CPS...worse things have befallen me.  I continue to noodle on the CEL.

A couple days ago I finally get around to plugging the vag-com cable 
back into the trunkputer permanently.  And now I notice that when I 
switch the machine on, the CEL goes out.  When the machine turns off, 
CEL comes back on.  And I don't mean in the Windows sense.  The light 
seems correlated to when the USB port has power applied, basically 
immediately after power-on while POSTing all the way up to power-off.

I'm confused as hell.  It seems that having the cable plugged into the 
car but without a running computer on the other side causes the CEL to 
come on, even if that computer were to be just staring at me with a 
blinking cursor.  It didn't used to do this and I've never seen any VW 
behave this way with any nationality of cable.  I guess I'll go out and 
like reseat some connections or something, but um...any ideas?  Any 
ideas at all?  Have I gone mad?


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