Even if the system will boot with a Q CPU -- which is unlikely, since the system never offered these -- the extra 10W of power required will quickly overwhelm the cooling system (which you cannot upgrade) and lead to crashing, instability, and likely quick system failure.

Hey, i just installed the Quad Core Q9000 processor and it seem's to not working. I think the problem is in the BIOS, any solution for that? I hear only CPU Fan starting and it turns off.

The P9700 is the most powerful CPU the e6400 (Intel PM45 /GM45 motherboard) supports. The T-series all have a 10W penalty. The P-series are the most efficient MHz/W. Combining the P9700 with an Intel SSD and 8GB of RAM, my e6400 Windows Experience Score jumped several points. That said, the Nvidia NVS160 GPU is the lowest score and the built-in Intel Graphics version of the motherboard would be even lower. FWIW, the P9700 is roughly the equivalent of an Intel Core i3. If you really need more power, you really need a new laptop.

Final thoughts: make sure to use copper shims and Arctic Silver 5 when re-attaching the heatsink to the GPU and FSB (the CPU is AS5 only). The heatsink "tape" is only really one-time deal. I think Notebook Forums was where I read the article about how to use the copper shims and AS5.

Do the Bios update and be prepared - it may or may not acccept the Q9000-

There are 2 types of motherboards in these:

Remember on these 6400 and 6500's there are 2 types: discrete and non-discrete video motherboards. going. One 6400 I have would not see the processor; also another Lenovo would not see it; so I used a 3.06 or a 2.93 Duocore/Core2Duo and kept on I have used the 6500 Lat with Q9000, without overheating problems. Though not a "stock" option, there is an upgrade for the cooling system; if you are capable: 

What I did on a Lat. 5400 that I have installed the Q9000 (2.0 x 4 thread) processor that I just bought for $22.00 (first one I bought 2 years ago was quite a bit more) - A piece of thin Copper sheet custom cut with shop sheatrs or dremel cutter obtained from your hobby shop installed between the heatsink and the screw-on heat displacement unit will help wick the heat out. I have used a US 1cent (penny) in some laptops to assist the system's capability to move heat. There are lots of laptops that (from the factory) don't come with adequate cooling for video card area which is where I have added the "penny" method.I have never had a laptop overheat yt the 6500 and 6400 line does spew out plenty of heat so you should take care to keep the fan free of any dust bunnies any time you take the back off or remove the fan assembly- blow through with a drinking straw (don't waste money on canned air).

The single best speed increase for the money is the 2.93ghz x2 processor and a Solid State Drive. Clonezilla is a free tool that will copy any drive to an equal or larger size drive- I use it reguarly and have 6 of the Lat Dell 6400/6500 - THere is also a USB3 card that these can take on the card slot (Express) for 15. but it ships from China (can take a while). 

Excuse me... but ,I think my level about hardware is very poor... for this reason I have a question Can you give more information of penny method?

I would also like to know more about this

6 e6400 series?  Man I need to upgrade mine a little bit.  It has a bad over heating problem. I woodshed I could play my games a lot better.  I play RimWorld Binding of Isaac and Noita. But lately Noita has not been running very good. 

@Gladis666​ 

Yoo it napped to me while I was fixing my laptop reseat the cpu ram or Wi-Fi card

I have had my E6400 for about 2  years.  I have done the following mods:

1.  RAM is now at 8GB - 2 ea 4GB DDR2 SODIMMs. If you have an E6400 it should be at 8 GB.  This is the max for DDR2 SODIMMs.

2.  I installed a 2 TB SSD (2.5" SATA format).  It has 4 primary partitions.

    Partition 1: FAT  2047.32 MB for PC-DOS 7.0 Revision 1 (AKA PC-DOS  2000).

    Partition 2: NTFS 80GB for Windows 7 64bit.

    Partition 3: NTFS  1109 GB for Windows 10 64bit and DATA

    Partition 4: EXT4 712 GB for Linux Mint Cinnamon 21.3 64bit

I also keep a 1TB SDXC SD card in the front SD card reader for data.  Next best thing to a real SSD.

3.  The E6400 came with the Intel Core Duo P8400 cpu - 2.26 GHz and 25w TDP.  Right away I installed the Intel T9900 Core Duo 3.06 GHz 35w TDP using the stock E6400  heat sink with no over heating issues whatsoever.  YMMV.

Linux Mint Cinnamon runs faster than W10 IMHO.  I still have a few apps that need Win7 and/or Win 10 so I keep them around.  Linux Mint is my main OS so this laptop still has a lot of life left in it. I find the built in DVD drive useful.

When I need something a bit lighter I use an Acer Aspire One 756-2899 net book with 8 GB ram and a 512 GB SSD.

@Gladis666​ no reseat it or try again it happened to me just do those steps

@Aoresteen​ What brand did you use for your ram upgrade ? Got link ? I googled it and it was fairly pricey. 90$ usd +

@moder000​   Sorry for the delay.  8 GB RAM modules are pricey and are getting harder to find. I now have 4 E6400 laptops and I just use what I can find.  I recently bought 3 sets of 8GB RAM DDR2 off of eBay for my 4th E6400 and got 2 sets for working spares.  Cost shipped was $291.44 from Star Galaxy Inc.  I only installed matched pairs and do not mix manufactures.  Make sure you are at BIOS A34.  The price will only go up.

I have 2 E6400 with the T9900 CPU and my 4th will get one soon.  The stock cooling heat sink & fan can IMHO handle the T9900 heat output.  YMMV.  It is very fast with Linux Mint 21.3 so I have never needed to try a Q series CPU (I have quite a few of them including a couple of Q9560 3.0 GHz ) in the E6400.  I would never ever try to put the Q9560 with 95 watts TDP in an E6400.  The T9900 IMHO is the best CPU for the E6400.

I like the E6400 because will run every thing from DOS to Win10 (I have never tried 11 on it but I might just try it) and Linux Mint 64bit.  Not to many other laptops that can do that.

I just threw away my'n with the nvidia setup a few weeks ago. But I only wanted it for a plex server or plex client. But I than realized the other day I tossed the charger a long time ago :(

But I now are going to use its monitor.

I fell in love with it wen I got it from a used reseller. It is a good one.