4lane

The chirp on locking the car with remote recently stopped. When the car is locked, all doors are locked, except the fuel door, which doesn't seem to lock. When driving above 10mph I can hear the car constantly locking itself. It clearly thinks it's not completely locked because in traffic it will continually cycle to lock all the doors once you go above 10mph. This would also explain the lack of chirp on locking - the car knows it's not fully locked due to the unlocked fuel door. I do have two fuel door codes, indicating the fuel door actuator may need replacing, but my question is this: If the fuel door does not lock, does the car think it is in an unlocked state?

2 Faults Found:
02615 - Fuel Tank Cap; Lock
009 - Open or Short to Ground
Freeze Frame:
Fault Status: 01101001
Fault Priority: 2
Fault Frequency: 2
Reset counter: 200
Mileage: 176871 km
Time Indication: 0
Date: 2018.04.10
Time: 21:07:57

02616 - Fuel Tank Cap; Unlock
009 - Open or Short to Ground
Freeze Frame:
Fault Status: 01101001
Fault Priority: 2
Fault Frequency: 3
Reset counter: 202
Mileage: 176871 km
Time Indication: 0
Date: 2018.04.10
Time: 21:14:10

SMac770

So if you open the fuel door, press in the rotating post that holds the fuel door closed so it is in the closed position (it should stay without you holding it), lock the car, then press on the post, it moves back to the open position?
Looking at the pics of the fuel door actuator, it appears to have just two electrical connections. I assume that to be power and ground. Not sure if "energized" would be to lock or release the solenoid though.

Your conclusion seems reasonable, but I doubt even AoA knows the product well enough to confirm it.
The other end of the two wire connection from the V155 motor is the J393 comfort control module (big component in the right side trunk area with the multiple large connectors). The T17o 17-pin connector (well, for a 2009 at least). Could try to unplug/replug it (with the car off).
Not sure if there's measured values blocks you could review in VCDS to confirm states. Still wouldn't confirm if it's the cause of the constant relocking attempts.

4lane

4lane

I opened up J393 to take a look at the board to see if I found anything obvious. I see four areas of oxidation of some sort. You can see two of them in this image, one circled. Does this oxidation potentially interfere with the circuit and J393's functions? And if so, is it possible to clean it?

SMac770

When my advanced key stopped working, Internet resources were showing how VW versions, it was just a transistor and resistor that needed replacing, typically. But I decided to just buy a used replacement on eBay for $100. Prob at that point is you have to have it recoded by the dealership (1hr billing) since the J393 manages a ton of stuff, including interaction with the immobilizer.

Without a counter example, you can't know if those marks are expected or even intentionally implemented.
I'm not enough of an electrician to know if cleaning that up would assist, or how to clean it without endangering the chips. But I would say it doesn't look great. Though whether it impacts operation, no idea.
So it could be your issue is other, maybe a component blew, and the locking of the fuel door is an exhibit rather than a cause.
Problems with the J393 due to moisture is something I ran across in my research, but it was not relevant to my scenario.

4lane

SMac770

Yeah, you can't recode that particular module yourself, even if you have VCDS. Otherwise, I would just have read the long coding off the old one and applied it to the new one and been on my way. Just note, there's a lot of revisions of that part number; check with the dealership the correct current and previous revision for your VIN before you go looking for one. Assuming replacing your J393 is what your scenario ends up coming down to.