Dell E7240 won't turn on

Thanks. Will try without the battery as well as without the keyboard, and will keep you posted.

To answer your question, I have never touched the CPU thermal paste of this laptop. I understand that CPUs come soldered to the motherboard, wouldn't this imply both motherboards have the same thermal paste issue ? You seem to have experience with this laptop, do you think this could this be because of a defective fan?

As a sidenote, I got a second-hand motherboard from ebay for €50 so probably still cheaper than the no HD option (which in hindsight would have been a better choice I agree).

@Jean Odoux I would not trust the paste they come with - repaste it. At least with No HD you could have transferred the ownership, swapped the drives, backed up and reinstalled 7 and upgraded it to 10. As a general guideline, my rule is if it's 2-3+ CPU generations out of date with a major issue (Ex: bad motherboard) or expensive work needed, I usually upgrade to at least a Haswell based system (Ex: Sandy Bridge->Haswell).

In your case, you're already there. In this case, what I'd look into doing is a "like for like" system replacement (7240 -> 7240) or a 2 generation CPU upgrade (7240 -> 7270) - but in your case you'd need new drives since the 7240/50 are mSATA based vs. M.2 NVMe based like the 7270. The 7270 uses Skylane with NVMe, while the 7240 is Haswell with mSATA. I don't like them for new owners, but in your case you own the drive it requires - good luck finding one if you're just buying one for yourself if you don't want a Chinese SSD at double the price.

I'm familiar more with the E6 series personally - I had an E6540/Radeon 8790M edition and it had to be retired due to the GPU burning out.

If you keep having issues with it yeah buy one without the drives, transfer the ownership (you either need a proof of purchase or need to know or it can't be done) and move your drives to the replacement. I'm in the US, so board swaps are almost never viable with how many ex corporate machines you have at your disposal to pick from to replace a dead board machine for cheap if you shop and bid right. The catch is you absolutely need to check if it's cam/no cam because a lot of the bigger companies order no cam machines since they can afford the delete, while end users, small order lots and one off buyers use tape.