No boost and can't scan OBD2?!

I have a bit of a puzzler and I thought I'd solicit some help. The other day I was driving my 2000 TDI Golf and I noticed I suddenly had no power. Boost gauge does not move. My exhaust manifold pressure gauge also does not move. I never heard a pop, but I checked for blown intercooler coupler for my front-mount and everything seems fine. Checked for a holes/cracks/leaks in the exhaust manifold and nothing obvious (I have a cast manifold with an adapter and a GTB turbo). So I get out the VCDS cable and laptop and it won't connect to any module. I run the test and make sure the com port is correct and it sees the interface just fine. I run the auto-scan and and every module times out. I looked at the OBD2 connector and there are no pins missing or pushed out, and I have the factory radio (but I did hook up an amp with the blaupunkt ISO adapter). I've used the very same VCDS cable and laptop before, so that hasn't changed... If the ECU died why does the car still run with no CEL (albeit with no boost)? If just the N75 died, how come I can't scan anything? Arrrgh! Anyone have any ideas?

Does the interface lamp illuminate when you plug in the interface? Is fuse #12 OK?

Yes, the LED on the cable lights up. I pulled every fuse on the dash side panel and looked at them (one at a time). None seemed blown.

No other ideas?! The battery was a bit low so I charged it for a while. Didn't help. Then I disconnected the negative cable for an hour or so, because there is no control-alt-delete. Also didn't help. Maybe there was an EMP? Solar flare?

You need to check the K wire for short to + or short to common.

I removed the stock radio completely and checked the voltage on the K line. it's 2.5~3.5V with the ignition off and 10.5 with it on. Same at the OBD2 plug. From my searching this is normal behavior and it is not grounded or hot 12v. I unplugged modules one by one to see if one was polluting the K line with garbage traffic. Cluster, ABS pump, ECU, comfort module. Still would not scan. Tried both of my ebay cables, which worked before, with my registered VCDS lite. In a last ditch effort I plugged in my Flashzilla, and it can talk to the ECU. Only codes were Immobilizer from unplugging the cluster, brake lights from using LED bulbs, and ABS, also from unplugging it. Nothing related to the lack of turbo function.

I took off an intercooler coupler and piping is perfectly clean inside, so turbo didn't die and puke oil everywhere. Exhaust manifold pressure gauge needle doesn't oscillate like it used to. Acts like a hole in the manifold but I inspected everywhere at least 3x with a mirror, and there is no ticking/loud exhaust leak.

Another odd thing I notice taking a test drive around the block, my McNally gauge starts out at zero when I start the car, but gradually climbs and after about a minute, it reads the max 30psi. Never did this before. It is using a separate pressure sensor, not the MAP sensor. It does this every time I leave the ignition on. Somehow the sensor is receiving a stray signal... through the engine? It's mounted in my race pipe. Or maybe through the gauge ground? This is so bizarre...

Read this. http://www.ross-tech.com/vag-com/aftermarket-radio.html You have to insert a resister and see if it gets hot.

Even though your flashzilla didn't get a code maybe you still have a boost related code. I might suggest getting vagcom working and scanning it while its in limp mode.

Thanks for the link. I followed the ross-tech procedure. Plugged factory radio back in, turned on ignition and radio and voltage was 10.5... "inconclusive." Next I put a 1K resistor between the 4&7 pins. Voltage across resistor was .5V and it never got hot. So... k-line isn't grounded or 12v.

Where is the airbag controller? I try and figure that out. Just realized I never unplugged that and tried to scan. I installed later MK4 seats and swapped over my early seat airbag wire harnesses, and it's been squaking at me everytime I turn on the ignition with an airbag message on the MFA screen. Can airbag problems put you in limp mode? The lack of boost didn't occur at the same time as the seat swap though.

Just thought I'd update the ongoing saga...

Check engine light came on for the first time the other day. Hooked up the flashzilla and tried to scan. No codes found.

Yesterday I disconnected the turbo vacuum actuator from the N75 valve and hooked it up to a mighty vac. Pumped it and the VNT lever moves, so the actuator works. I ran the line into the cabin and went on a test drive. I gave it some vacuum and then some throttle. No power. A little more vacuum, still no power. My fear was that my weld on the VNT lever inside the turbine housing failed. If only the boost gauge wasn't out to lunch this would be a good test of that. At first I assumed no power=no boost=broken weld, but I realized that the ECU may be outsmarting me, not giving any more fuel when there is boost, because the ECU may not be asking for boost. Looks like I need to redo the test with a mechanical boost gauge that actually works, to see if the manifold will at least pressurize.

If it turns out that I can't make the turbo spool with the mighty vac and the VNT lever is indeed broken inside the turbo, I guess I have multiple problems, because that still doesn't explain the inability to scan any controllers.

dont forget to unplug the engine computer and see if other modules will communicate. I had a audi a4 that the engine computer was making no communication with any module the other day.