Oldie but Goldie... With Strange

[Guest]

Hello, I have a Dell M6400 that was working fine, then stopped (just died). I switched it back on, and it did not boot; instead, the No. 9 bag (Num Lock) led flashed 23 times and then remained on. According to the Dell Precision™ M6400 Service Manuals, it signifies a processor error.

So here is what I have tried so far.
1) Removed and cleaned the CPU, reseated it, applied new thermal paste and retried and got the same error.
2) Purchased a replacement CPU, applied new thermal paste, and retried and got the same error.

Now, thinking this is not the processor, I purchased a replacement motherboard and tried both CPUs, but I still get the same error message.

So please, any suggestions...

[Wasmachineman_NL]

Precision mentioned. Tried different memory?

Could also be a dud GPU, which these are not uncommon for.

[Trebor58]

Thanks, Wasmachineman_NL, would the error code not show Graphics error instead of Processor error

[Guest]

Here are the CPUs I have been trying...
Intel Core 2 Duo P8700 2.53GHz PGA578 Laptop CPU Processor
Intel Core 2 Duo P8400 - 2.26GHz Dual Core CPU Processor

The sockets on the two motherboards are 479M; the socket on the working system, according to CPU-Z, is 478 Socket P. Google states that the M6400 was only shipped with a 479M, but I have always installed a 478 socket.
So, I am going to try and locate a socket 479M CPU, all I can find isa 479 CPU, not a 479M