ROOMBA·FAULT ERR 21 WARN
Roomba Error 21 — Internal communication error
Severity: Warning — mission stops; usually clears with a restart or reboot ijs
DIRECT ANSWER
Error 21 means an internal communication error — one of the robot's boards stopped answering. Press CLEAN to retry. If it comes back, reboot. Error 21 that survives a reboot needs iRobot support, not parts.
#meaning · #causes · #fix — link straight to any section
COMMON CAUSES · RANKED
1
Momentary firmware fault MOST COMMON
A hung internal process drops communication once and never again.
2
Board-level hardware fault UNCOMMON
Repeated error 21 after reboots points at a failing electronic component inside.
HOW TO FIX IT
No battery or electrical work involved in this fix.
Press CLEAN to retry
Most occurrences are one-offs. Restart the mission and watch whether it completes.
Reboot the robot
Hold CLEAN until the light ring swirls, then let it boot fully before testing again.
Contact iRobot support
If error 21 persists after a reboot, the fault is internal. Note your robot's serial number before calling.
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Corrections
Spotted an error? Report a correction — fixes ship within 48h and are logged on this page.