I have a 2001 9-5 that has exhibited a problem for maybe two or three years now: It sometimes fails to start (won’t even crank) after a short drive and sitting for about an hour.
I drive to a pool almost every morning, swim for about an hour, and then drive back. It always starts fine first thing in the morning, but once in a while it won’t start after swimming. When it fails to start, if I wait about 30 minutes, it then starts fine, as if there never was a problem.
It happens maybe once every few weeks. The dealer has never been able to get it to reproduce, nor to find any fault. If I replace the battery anyway, the problem doesn’t happen again for 6-12 month. There’s never any indication that the battery is at fault (it always tests fine), but replacing it seems to help for a while.
I have a radar detector plugged in to the cigarette lighter, and it displays the voltage. When the car fails to start, the display is generally just below 12v (maybe 11.8 or 11.9). Once it starts, it’s well above 12.