This problem is almost (if not actually) identical to one posted here 3 1/2 years ago by another forum member, but she received no reply. This is a GSD 2800 L03 built-in dishwasher, installed 1990. It's seen the repairman several times over its life.
I get a C2 blocked drain code when I run the dishwasher. This has been going on for well over a year, but it usually only happened after I filled the rinse agent dispenser, curiously enough. Now it is happening constantly, which means I cannot get through an entire wash cycle - only the first five minutes before the code kicks in. I then hit the "clear/reset" button, but that also does not clear the code (keeps running), so I have to turn off the breaker switch to the dishwasher to stop it.
Now, I have thoroughly and assiduously examined and reamed out EVERY connecting hose and drain and checked the backflow flapper valve - you name it, I"ve checked it. There are no obstructions whatsoever in the machine, yet it continues to deliver this error.
There is no air gap, but there is a high loop to the drain hose that is higher than the sink drain. I've also checked that hose to make sure there is no kink at the high end.
I am now beginning to think it might be a bad motherboard. This was replaced about three years ago, and I think it has a five year warranty. I'll entertain any ideas. I'm desperate for a solution.