Put this into the Latitude forum as my machine is functionally similar to the Latitude E6400 and I can't post in Laptops General.
I recently replaced the stock Intel Core 2 Duo T9800 in my laptop with an Intel Core 2 Quad Q9100, which I can confirm works. After upgrading the CPU and pressing the power button, the laptop stays on for 5 seconds and then shuts off. Nothing is ever displayed on the LCD, and there are no lock error codes. If I replace the old CPU, it works again. I also tried a spare Core 2 Duo T9400 and it exhibited the same problem. The BIOS version is A29, the latest. What can I do?
It's difficult to say - but this is a ten year old system. Make sure the CPU is securely locked down. For the quad-core, you may be seeing the effect of aged mainboard components that can supply the 35W needed by the T CPU but not the requirements of the Q CPU,which draws 45W.
It's not clear why the T9400 isn't working -- has that been tested on another system to verify its status?
Yes, it previously worked in a ThinkPad T400.
Just now I tried plugging in the AC adapter. With the Q9100, it stays on for 15 seconds and then spits out a Flash-On-On lock code. With the T9400 inserted, it turns on properly and I can access the BIOS. System logs say that there were two power events with "CPU" for the power down reason.
That's a no-processor detected. If you know the CPUs are OK, but two don't work - I'd bet it's a mainboard starting to fail from old age.
I doubt it's a mainboard failure. I've gotten the T9400 to work by plugging in the AC adapter and letting the laptop "check" itself. I'm thinking that the Q9100 is a dud.