On a recent occasion, one student's MacBook Pro started up with three beeps and showed abnormal behavior. Using Apple support guidelines, we now know the meaning of the startup beep codes. On a MacBook Pro, the startup POST (power on startup test) checks hardware functionality. The following beep codes indicate a variety of issues. Here is the listing provided by Apple technical documentation.
On MBPro Startup:
1 beep = no RAM installed
2 beeps = incompatible RAM types
3 beeps = no good banks
4 beeps = no good boot images in the boot ROM (and/or bad sys config block)
5 beeps = processor is not usable
One beep seems self explanatory in that there are no RAM SO-DIMMs installed in the machine. While this is not likely to happen in many cases, you will quickly know the issue.
Two beeps on a MacBook Pro indicate that the RAM SO-DIMMs are not compatible. For instance, if the memory specification requires PC3-10600S DDR3 1333 MHz RAM, that is what should be procured and installed in the MacBook Pro. Not all RAM modules are up to Apple's specification and it is wise to purchase RAM from vendors who explicitly state MacBook Pro compatibility.
Three beeps indicates a tricker predicament. The problem is most likely not RAM compatibility despite the results of Google searches. The possible problems are more likely to be a problem with the RAM seating in the module banks. It is also possible that the circuitry of the RAM banks in conjunction with badly seated RAM modules has cause a memory management issue. Reseat the RAM modules and also reset the PRAM (parameter memory) on the MacBook Pro.
A POST with four beeps is a bit more serious. The ROM instructions for booting your computer will not function to startup the computer. Perhaps the ROM was corrupted by some electrical damage involving the ROM chip and circuitry. Generally, this problem will require a motherboard replacement.
Finally, five beeps also indicates a fatal problem. This failure is found in the central processing unit of the laptop. Again, damage of some type has occurred with the CPU of the MacBook Pro, and will require a motherboard replacement in most cases.
In the case of student laptops at North Mahaska, the problems seen so far have been minor. One laptop did come in with the 3 beep POST and a RAM reseat and PRAM reset seems to have fixed the problem. Otherwise, one hard drive and one trackpad in 300 laptops has failed in six weeks. We continue to monitor the laptops with JAMF Casper and ARD software products. If the hardware continues to be solid, then we can turn ourselves to software issues, which consume more time due to complexity relationships in installation and usage.