The MechaCon (Mechanics Controller) is the chip inside the PS2 which controls the console's disc drive. It also handles some security functions.[1] This crash is improperly named as it is actually the DSP (Digital Signal Processor) that crashes, not the MechaCon. This happens when the DSP receives bogus ECC (Error Correcting Code) from the disc due to scratches, bad burn etc. If the DSP crashes while it was sending a pulse to the laser diode, the laser may burn up. This may happen with original or burned media.
When using MechaPwn or a modchip on an affected model, it can be a good idea to use one of the fixes on this page to prevent possible damage to your console.
For more technical details regarding the issue, take a look a the PS2 Developer Wiki's MechaCon and DSP pages.
Why it happens
The MechaCon works together with the DSP, another chip inside the console used for signal processing. Some firmware revisions of the DSP can cause the MechaCon to crash.[2]
Whether this was intentionally done by Sony is debatable. Some argue that it was an attempt by Sony to combat modchips, however there is no proof this is the case.
Affected Models
During the PS2's lifespan, 2 MechaCon versions were used. A SCP970 based one, used until the SCPH-390xx, and the ARM-based "Dragon" chip used on SCPH-500xx and up. Mechacon version is irrelevant.
While the DSP with the firmware issue was already used on 390xx consoles, the crash is far more common to happen on the Dragon models, so SCPH-500xx and SCPH-700xxx (last Fat models and earliest Slim models).[3] Note that the SCPH-700xx models can sometimes have a newer DSP firmware with a fix, but not all of them have it.
Later Slim models from SCPH-750xx and up have fixes for the problem from the factory.
Fixes
Numerous fixes for the problem have been created over the years, some being more effective than others.
PIC Fix
The Matrix PIC fix is the only fix found to actually save a console in the event of a crash. It monitors the communication between the MechaCon and DSP and shuts down the console when a crash is recognized.
It uses a PIC (programmable integrated circuit), which are really cheap and run a very simple software.
The PIC fix is effective and cheap to implement. Check out the MechaPwn team's research and write-up. Optionally, you can get or make a PCB for a cleaner installation, like the one quick solder boards by PS2 Modchip Tutorials or by ModzVilleUSA.
Diode Fix
There is also a "diode fix", however it's not effective according to the MechaPwn team's research.