I recently acquired an old Dell Latitude E5500 laptop as a workplace giveaway. Below are the original specs:

Intel Pentium M 575 2.0 GHz
2 GB RAM DDR 667 MHz
160 GB SATA HDD

I had to replace the battery with a new one and wanted to upgrade the processor to Core 2 Duo. I bought a Core 2 Duo P8400 2.26 GHz processor, but for whatever reason it's not recognized fully by the BIOS. I upgraded the BIOS to version A18, though the same issue persists. The laptop starts, but then instantly shuts down. No error code is displayed and none of the diagnostic LEDs light up. This is not the same error as a missing or incompatible processor, as I have tested this with a Core 2 Duo T7200.

I know this model can be used with the P8400 or even stronger processors, hence I'm completely confused why this is not the case for me :/. Unfortunately, the Dell website does not mention specifically what is required for an Intel Celeron M to Core 2 Duo upgrade for this laptop model.

Any help would be greatly appreciated.