ROOMBA·FAULT ERR 21 WARN

Roomba Error 21 — Internal communication error

Q: Roomba Error 21 — Internal communication error

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.

Severity: Warning — mission stops; usually clears with a restart or reboot ijs

Last verified: 2026-07-05 Tools: none Time: ~5 min Difficulty: easy

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 · #models — link straight to any section

COMMON CAUSES · RANKED

Momentary firmware fault MOST COMMON

A hung internal process drops communication once and never again.

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.

Did this fix your robot?

No reports yet for this fix — be the first.

MODELS AFFECTED

Model	Status	Firmware	Parts	Verified
Roomba i1 / i1+	limited	i	Parts →	2026-07-05 · Source
Roomba i3 / i3+	limited	i	Parts →	2026-07-05 · Source
Roomba i4 / i4+	limited	i	Parts →	2026-07-05 · Source
Roomba i6 / i6+	limited	i	Parts →	2026-07-05 · Source
Roomba i7 / i7+	limited	i	Parts →	2026-07-05 · Source
Roomba i8 / i8+	limited	i	Parts →	2026-07-05 · Source
Roomba j7 / j7+	limited	j	Parts →	2026-07-05 · Source
Roomba j9 / j9+	active	j	Parts →	2026-07-05 · Source
Roomba s9 / s9+	limited	s	Parts →	2026-07-05 · Source
Roomba Combo j7+	limited	j	Parts →	2026-07-05 · Source
Roomba Combo j9+	active	j	Parts →	2026-07-05 · Source

Models resolved from the series chips above. Support status comes from our EOL library; rows link to the matching parts and firmware pages.

Sources

Verification

OFFICIAL SOURCE

Last verified: 2026-07-05 · How we verify a fault code

Corrections

Spotted an error? Report a correction — fixes ship within 48h and are logged on this page.