Macbook automatically shuts down after booting up

Hello all, I'm a new user here, although I've been a macbook user and lurker here for the past six years. Well here's my problem: My macbook, the 2008 unibody model, automatically shuts down after it starts up. I've done some research and found out that the problem may occur for a variety of reasons, but there's no definitive solution or fix that people have discovered. So basically, I've reset the SMC and the PRAM. I've done a full reinstall of OSX Snow Leopard and even disconnected and reconnected the keyboard ribbon, a suggestion I received from another discussion on the issue. So far nothing has worked. Several people believe the problem occurs because of the magnet senors on the cover. In oder to replace them, I will have to buy a full top cover, which costs more than half of what the macbook is worth right now, and I'm not even sure if that might be the problem. I don't think it's the logic board since the computer works when it's plugged into a monitor.

Please help me if you have any suggestions. Since my macbook is so old, Applecare is out of the question and I don't think it's worth shelling the money for a logic board or cover repair. A better solution would be to sell it for parts and move onto my next computer, which I want to avoid since the computer still works fine with a monitor. I am desperate for help!

From your fault description it sounds as if the MacBook isn't shutting down, but looses it's video or backlight on its LCD. By running OK with an external display connected this would tend to suggest the display was failing, but there is also a smaller chance of a logic board fault. This is because your external video via the mdp and the feed to the MacBooks LCD are fed from different parts of the logic board video circuitry. The only sure fire way to determine what is failing is to temporally connect another known good display clamshell. I can do this at work, but you won't have access to the part. I doubt if you have any friends who have the same MacBook that would let you pull theirs apart to aid a fault diagnosis. It's looking as if selling it for parts may be the best option, or if the MacBook is desk bound, continue to use it with the external display.

Thanks for the reply, techiesteve. The macbook is actually shutting down once I turn it on as indicated by the status light on the front and the noise that the disk drive makes every time I try to start it up again. I really wish I could go over to the UK so you can take a look at it, but as you said, the best option may be to sell it for parts -- they are still worth a lot of money, surprisingly. I had this computer for six years and I got very good use out of it; it just surprises me that it would break down so suddenly. It was working fine yesterday. I was hoping there was one more fix or solution left before I had to say goodbye to my old friend

I finally figured it out! It was a sensor error. The hardware test gave me a code for a sensor error, although it didn't give out anything specific. So I did some research and found out that my ir sensor was the culprit. The sensor somehow communicates with a mechanism called the "reed switch" -- it reads the LCD magnets when the clamshell is closing. My laptop kept shutting down because the reed switch or the ir sensor kept giving the signal that the case was down. What I basically did was open up the back of the laptop and I took out the optical drive. Here are the instructions I followed: MacBook Unibody Model A1278 Upper Case Replacement - iFixit

Step 33 will show you guys the ir sensor. There was some black stuff on the end of my sensor so I basically just cleaned it off with rubbing alcohol. I also saw that the capacitor, or battery, that is connected to the logic board, right above the trackpad, was corroded. It's called the PRam battery. My computer runs off the main battery already so I had no need for the corroded PRam thing. I took it off, too. I also took my ram chips off and put them back in (a tip that a mac technician told me to do). I did this all at once so the solution could've been any one of them. Most likely, however, it was the ir sensor. I should replace it, but I'm going to run what I have right now until I run it into the ground! And to think, I was ready to sell the whole thing for parts.