I have a 6-year-old, self-built, AIO water cooled PC (specs below) that for various reasons has only been in regular use for the past 3 years. For those last 3 years, there has been no change in configuration and things have been running smoothly (successful stress test done a year ago). Two days ago, I received a "CPU Fan Error" and “CPU Over Temperature Error” on bootup and on subsequent examination, the CPU was found to be running consistently at 99-100 C at idle (rapid linear increase). SPECS: Cooling (liquid): Corsair H100i v1 CPU: Intel i7-4770K MoBo: Asus Maximus VI Formula PSU: Corsair HX1000 Case: Corsair Carbide Air 540 From my reading, the possible reasons for this "CPU fan error and overheat" could be: a) Cable connection issues (to the PSU and/or motherboard) b) Thermal paste has dried up c) Poor seating of CPU cooler and/or dust clogged exhaust fans d) Failure of the AIO water cooler What I did: A) Cable Connection Issues: Since the configuration has not changed and the computer has been running smoothly till now, I assume that this cannot be the issue. In any case, all connections were found to be secure (including the three pin "CPU_fan") and I even changed the SATA connection to the PSU with no improvement. B) Thermal Paste Drying: Old thermal paste was removed and replaced with fresh paste C) Poor Seating or Clogged Fans: Seating was confirmed to be solid; exhaust fans (horizontal, located at the top of the case) are visibly working and dust free. Mild vibration in pipes is felt. D) Failure of AIO Water-cooling: When the status of the H100i was checked using Corsair's Link application, it showed coolant temperature at 70-80 C (on average), pump fan speed at around 2500 rpm and exhaust fan speed at around 2700 rpm. After all this, I still am receiving the same errors. So, my question is how should I proceed? Everything seems to be working! Despite this, the CPU is not being cooled. A) Cables seem secure. B) Thermal paste has been replaced. C) Despite the CPU fan error thrown up, the Link software shows both the pump fan and exhaust fans are running. In addition, the high temperature of the coolant would seem to indicate that it is taking up the heat from the CPU. Is it sufficient to say the water cooler is working because the fans are working? Could the fans be working and the water cooler still not doing its job? Any way to check that? D) Additional Question: The CPU is currently running at 100 C at idle/minimum load, as soon as Windows loads. Is that solely because of the lack of heat removal or could the CPU itself be at fault? Shouldn't a CPU at idle run at a much lower temperature even in the absence of cooling? Finally, where can I find the fan settings in BIOS? I cannot seem to locate them. I do not want to buy a new CPU cooler only to find out the problem is something else since the refund/return policy where I am is non-existent. Any help is appreciated. If any other information is required, I will try my best to provide it. Thanks for reading.