Hi there. I have 'solved' this, but I thought it was worth posting in case anyone else finds it useful. I've had my One X (32gb, Quad Core, Tegra 3 version, standard OTA 4.2.2 Android) for just over a year and apart from average battery life and weak WiFi I can't really fault it. But this last week I suddenly had an issue where when I turn the phone on in the morning - I always turn my phone off at night - the battery was flat. I had always used the phone on the T-Mobile network here in the UK, and last Monday I changed over to Tesco Mobile (o2 network) and straight away in the morning, flat battery. As the only thing I had changed was the SIM I thought that must be the culprit, so the next night I removed the SIM and turned off the phone as normal, but no, still a flat battery in the morning. Next night, I turned off Fastboot, and bingo the battery was fine in the morning. Just to test it again, the next night I turned Fastboot back on and again a flat battery in the morning. The built in battery monitor suggested that the two things using battery whilst the phone was turned off was the WiFi and Mobile signal (oddly) so last night I turned both of these off and enabled Fastboot again, but this morning, flat battery. So I have my solution, which is just to deselect Fastboot....it doesn't seem to make that much difference in boot up time anyway. I understand that with Fastboot enabled the phone doesn't entirely turn off. But why the difference between network providers, I have made NO other changes to the phone. I can't put my old SIM in to see if going back to T-Mobile puts it right again because I ported my number over, so the old T-Mo SIM is essentially 'dead'. Any ideas?