Fix Neff Dishwasher?

I have a Neff SH4443N6GB dishwasher. It lsuddenly eaves glass grey & things not very clean. Works better on 50degs than 65. Salt & rinse-aid OK. Cleaned filters & noticed a little black grease round the filter holder inside. Tried dishwasher cleaner, but the wash didn't even melt the top to let the cleaner out of the bottle, so maybe a heat problem? it does get hot, though can't tell it enough. Any ideas? How do you get at the thermostat, heater, etc and how do you test - I can use a multimeter. Can I repair it myself?