I have a Z5 and today, the black screen bug occurred for the first time. It really bothered me and seems to suggest that Sony quality is deteriorating as my Sony a5000 camera also had some problem which caused it to lock up up. It's my first Sony camera but I have used Sony phones since the Sony-Ericsson days and never had problems. The quality was so good that I remained loyal right up to today.
Anyway, after reading many different posts on the black screen thing, I noted that many people indicated the proximity sensor was the culprit. So I studied my proximity sensor at the top (adjacent to the front camera) closely and saw that it was covered with dirt. I thoroughly cleaned it and then tried making calls and it was working again like normal! Perhaps people who tried this solution didn't realise the sensor area got dirty again and the problem came back. But I am hoping it is nothing more than that.
There is some solution where the proximity sensor can be switched off and I might try it but it involves going into the service diagnostics area and I would rather not go there unless necessary.
Hope this helps.
Chips
Malaysia
