F2 error, wash cycle stops and control pan...

Hi, I have an asko 1875 that stops during a wash cycle, reports error F2 and flashes all the lights on the front panel. If you press start it pumps out the water, fills and washes again for little while before repeating the problem. The countdown timer seems to remember where it's at as after around 8 restarts the cycle ends but the soap dispenser never opens and the dishes never get a proper clean. There was a problem with water in the bottom but this was removed 4 days ago and hasn't come back. It's worth noting that the bottom is bone dry! After removing the water the dishwasher worked fine for a cycle but the problem has since returned. I checked the styrofoamm water level switch and it seems clear of the micro switch as you can clearly hear it click when you lift the styrofoam block up, and click when you release. It has just occurred to me that one thing has changed since it ran a successful cycle which is the adition of a drain hose extension. Perhaps this extension is causing too much resistence? It pumps out absolutely fine and unobstructed directly into the sink (not an s-bend water trap) at the beginning of the cycle. Anyway, i'll try it without the extension but if anyone has any answers why this error might be coming up for other reasons please let me know. TIA.