Fan specs and how to replace it

The HP Spare# for the 840 G2 is 730792-001 -- look for the original HP part, not a clone fan - the clones sometimes do not have proper PWM monitoring which freaks the HP BIOS out, even if the fan works but at full speed. You can use that part number and find one from a trusted vendor.

Here is the HP SM: https://h10032.www1.hp.com/ctg/Manual/c0... -- these do not require heatsink removal like many machines do today, but given the age of it the machine will likely benefit from being repasted too. If you are near a Best Buy, check there and get the Corsair paste: https://www.bestbuy.com/product/corsair-...