So today I noticed underneath my coolant reservoir there was a cable and plug hooked in neatly with all the other plugs that are clipped to the firewall. To my surprise, it wasn't plugged in to anything, and I don't remember any time I have unclipped, or even seen a plug like this. I ran my car after I found this, and didn't have any CELs, but I haven't scanned it with my VAG yet (It's midnight and I just finished working on the car all day, I don't have the energy to plug it in tonight).

Anybody know what this is for? Where it is supposed to plug in to?

 

Looks like the coolant level sensor, probably got disconnected when you removed the coolant tank

 

Nope, if you look at the first picture the coolant level sensor is in the bottom left.

 

Follow that wire because what you have makes zero sense. That's the primary O2 sensor plug, sensor side. But you already have a primary O2 plugged in and clipped to the second firewall.

 

Ohhhhhhhh! Nope, that makes perfect sense. I'm a total schmuk. A couple years ago I was getting some codes for O2, and so I pulled a sensor from a wrecker. Instead of going through and completely removing the old wiring (which would be a total PITA), I put the "new" sensor in and just wired that around and tied off the old one so it won't go anywhere. I completely forgot that I did this... Sorry for the pointless thread!

Thanks for the information Mad Cow. That is quite impressive that you can identify a plug just like that.