It is now time to clean the CPU heatsinks.

Here you can see the old paste on the occluding surface of the heatsink from CPU A. You can also see the dust inside the heatsink.

Blow out the dust from the interior of the heatsink, paying attention to the fans. Then clean the occluding surface and the surrounding skirt with Thermal Material Remover followed by Thermal Surface Purifier.

When these heatsinks are clean return them to the lid, out of the way.