I offer this for comments by the august population here at LL.
I had this sensor cleaned by Olympus, FOC. Yesterday, precisely two weeks after despatch, as promised, the camera returned but packed a lot less thoroughly than my own paranoid effort. When I opened the box and wrapping I noticed that it was stone-cold, and later discovered from the courier's website that it had come by air from Portugal. I did a quick check that it powered up, fitted a lens, set a couple of the operating parameters (it had been updated to firmware v 1.5) and put it in my bag for later use on a walk. In fact I didn't take it out again for nearly 5 hours when I had returned home.
I then took a few shots of the yard of a property I'm working on at which point the camera began misbehaving severely. Intermittent faults like freezing in various modes, sometimes recoverable by powering off and sometimes requiring battery removal. Failure to power-up or -down without a long lag; semi-powered states which displayed only a grey screen and no functioning controls. Audibly noisy start-up and power-down (when it would) etc. I tried 3 different lenses (Oly and Panasonic) with varying degrees of unpredictably repeatable fault conditions - or none.
I immediately completed Olympus' service inquest web form. Having sent them a faultless camera with a dirty sensor I'd received back one with a clean sensor but completely unusable, so the scores were on the low side. I also emailed Olympus' customer support (who I have found to be quite responsive in the past) with a description of the symptoms at that time.
This morning I started trying to find a repeatable set of conditions where these faults could be reproduced. And of course the camera refuses to misbehave. However shooting a capped lens I notice what appears to be a multitude of hot pixels, particularly apparent on a small JPEG (levels tortured a lot to show them of course.) This is much less apparent on a full size tiff generated in LR from the raw files.
My conclusion from all this is that it has been caused by moisture condensing on the very cold camera, despite the fact that it spent several hours in the dry and moderately cool conditions in my backpack. I'll attach the SOOC small jpeg in a subsequent post if anyone would like to examine it. My quandary is what to do now? If anyone can suggest a good diagnostic as to the general condition of the sensor I'd appreciate it.