One common issue that we hear about — “faulty MagSafe adapters” — stems directly from purchasing the new ones for pre-2009 machines. In certain cases, the adapter will not power the machine at first, and thus, appears faulty.
The solution is yet another example of why you should always keep your software up-to-date.
Pre-2009 Mac portables use the original style MagSafe adapter, which is a small rectangular block at the end of the cable. In 2009, it was redesigned to the current form factor — the “L-Shaped” design. If you try to charge a pre-2009 machine with this adapter, and if the software is not up to date, it will not work. This is because the SMC, a micro-controller on the logic board that controls the power functions on the computer, does not know how to use the new adapter.
Fortunately there is a free software update called “SMC Firmware Update.” To install the update, go to the (Apple) menu at the top left of your screen. Select “Software Update” and install all updates. Note that if you are not able to power the machine on, you will need to use a charged battery or find a pre-2009 MagSafe adapter in order to install.