my hp probook 640 g1 keeps turning off while in use

[o9ine]

my pc keeps shuting down while in use i changed  the fan but still shuys down

[Jupitero]

Hello.

How long does the computer work until it shuts down? Does it matter what you do with it?

The usual reason for computers to shut down suddenly is a thermal shutdown or faulty electronics. I recommend monitoring the CPU and system temperatures with e.g. Coretemp or HWMonitor software and see if rising temperature correlates with the laptop powering off.

If you took out the heat sink assembly while you replaced the fan, you should have also replaced the thermal paste - especially if it had the original, now 10-year-old paste in there. Thermal paste is available from all electronics boutiques and isn't expensive.

Also, take out the battery and run the laptop only with an AC adapter, preferably not the one you are using while having problems. If the power supplied to the laptop is not steady enough, the laptop may turn off. A decade old battery could exhibit odd behaviour.

If the reason for the laptop shutting down is not thermal related, nor due to the battery/AC adapter, then I would estimate the problem to be in the system board. Replacing it is possible but it is rather expensive part and probably not worth it unless your 640 G1 is really critical to your work.