The messages that appear before Windows loads are the most honest diagnostics a PC gives — the machine has already tested itself and is naming the part. Learn to read them.
Boot errors frighten users because they arrive before anything familiar appears. Read them literally and they are the friendliest faults in computing: the firmware has run its checks and is telling you which part failed. A SMART warning is the disk reporting on itself. A fan error is the board reporting a fan it cannot see spinning. A checksum error is a five-rupee battery announcing its retirement. This guide translates the common boot messages one by one — what each means, what to do first, and the one message where the correct first action is not a fix at all, but a backup.
👆 Example boot messages with their literal meanings. Boot text varies slightly by maker — the meaning does not.
This is the disk's own health system (Self-Monitoring and Reporting Technology) announcing that the drive has crossed a failure threshold. It is the one boot message where fixing is the second job:
Believe this message even when the PC seems to run fine afterward. SMART warnings are the disk confessing; disks do not confess for fun, and they do not recover.
CPU Fan Error means the motherboard cannot see the CPU fan spinning at the expected speed — the fan has stopped, its cable is unplugged or in the wrong header, or a replacement fan spins below the board's alarm limit. Open the case and watch at power-on: a fan that never moves gets reseated or replaced before the machine is used, because the board is protecting a processor that would cook in minutes. A fan spinning happily while the error insists usually means it is plugged into a case-fan header instead of the CPU header. The CMOS checksum error and its early cousin — the clock and BIOS settings resetting after every full shutdown — are one fault: the CR2032 coin battery on the motherboard is flat. It costs very little, takes minutes to change, and the only real consequence is re-setting the date and any custom BIOS options afterward.
This message means the firmware looked for something to boot and found nothing it could use. It splits three ways, and the split is quick:
The timing is a clue by itself: this error the morning after the desk was moved means connectors; after a BIOS reset or battery change it means boot order and mode were lost with the settings; on an untouched machine it leans toward the drive.
Keyboard error or no keyboard present is the PC asking for the one device needed to press F1 — reseat the USB connector or try another port; on old PS/2 systems, a bent pin is common. Memory size changed or memory test fail after the case was opened usually means a RAM stick was knocked partly out of its slot — reseat with power off. Insert boot disk variants are the no-bootable-device family above. Behind all of them sits one habit worth breaking: pressing F1 to continue, every day, forever. F1 gets you in once so you can work — but a boot message that repeats is a fault filing a daily report. Read it, name the part, fix the part, and the morning ritual disappears.
See every boot message on real machines in the video — SMART warnings, fan errors, dead CMOS batteries and boot failures, each translated and fixed.
▶️ Watch Full VideoNo — normal behaviour after a SMART warning is borrowed time. The drive's own monitoring has crossed a failure threshold; it may run for weeks or die today, and it will not heal. Back up immediately, then replace the drive on your schedule instead of its schedule.
The CMOS battery — a CR2032 coin cell on the motherboard that keeps the clock and BIOS settings while the PC is off. It is one of the cheapest parts in the machine and takes minutes to replace; afterwards set the date and check the boot order once.
Probably not — a listed disk with this error is usually boot order (something else is first) or a boot mode mismatch between UEFI and Legacy. Correct the order, match the mode to how Windows was installed, and most of these machines boot immediately.