The temperature sensor overreads by at least 3 degrees (sometimes more like 5), as far as I can work out.

If your unit is getting overly hot, the first thing you will notice is either (a) stuttering in tunes (caused by the discs retrying bad reads) or (b) intermittent player reboots due to (a) - the actual player will run much higher than 55, but the hard disks tend to get upset. I suspect the operation not permitted error was unrelated to any temperature issue.

A couple of users have placed 12v fans above the sled, blowing into the top slots of the unit - this will keep the temperature down. Not all cars have room above the unit though, though a fan underneath will also help a bit. Some cars are worse than others for heating issues, depending on what else is around the DIN slot and if there are any air gaps.

Hugo