I had a crash and my rear derailleur stopped working. How do I reset it?
Your rear derailleur has probably gone into crash mode. This means the motor has disengaged from the cage, blocking any further shifting. In order to restore normal operation, first make sure your rear derailleur and derailleur hanger are not damaged.
10-speed and 11-speed Di2 crash mode reset
To reset crash mode on 10-speed (6770) and 11-speed Di2 (6870, 9070, R8050, R9150, GRX), you get on your bike and press and hold the button on your junction box for 5 seconds until a red light flashes.
The rear derailleur will now go through all the gears, so make sure sure you spin the cranks until it’s done. Once it has gone through all the gears you’re done and ready to get back to enjoying your ride.
12-speed Di2 crash mode reset
Resetting crash mode on 12-speed R8100/R9200 Di2 works slightly differently. Since the button is on the rear derailleur itself, you cannot press that while riding.
Instead, you first shift to the largest sprocket, and then all the way to the smallest sprocket. That should reset the rear derailleur crash mode.