What is the deal with Shimano Di2 rear derailleur crash mode?
Troy's Tech Talk
It seems to be a bit of a hidden feature for most cyclists that even some bike shops forget. Shimano’s Di2 electronic drivetrain has a protection mode that it defaults to when it thinks there has been a crash. I say “thinks,” because I have had multiple instances where it just got bumped and went into protection/crash mode. I have also had countless real crashes where it didn’t go into protection mode at all.
What is really happening is, the Di2 derailleur motor disconnects from the cage in an effort to minimize damage. This leaves the derailleur in a state of no response when trying to cycle through the gears. This is easily remedied with a long hold (five seconds or more until you get a blinking red light) of the button on the junction box. Make sure the bike is in a stand or in a position for you to rotate the drivetrain. Also make sure that before you attempt this that the derailleur hanger isn’t damaged or bent. The red light on the junction box will blink, and then the system will attempt to shift through all the gears, so be prepared to spin the pedals so it doesn’t bind.
That’s it, a simple fix. Now, remember that this doesn’t mean the derailleur is safe to use. It only means that it’s out of the protection mode and ready for further assessment for damage. I can’t tell you how many times someone has come into the local shop with a new derailleur that they just purchased to find out it was just in protection mode. On the flip side, I have purchased a few broken Di2 derailleurs for parts, and it turns out they were just in protection mode.