A collection of less common but important iPhone errors and alert messages — covering miscellaneous hardware failures and post-update issues.
Error 21 — DFU hardware error
The device can't enter DFU mode properly, or security software on the computer is interfering. Try first: Disable antivirus/firewall → try on a different computer. If that doesn't work, the device may have an internal hardware fault preventing DFU mode.
Error 40 — Internal component communication failure
Something inside the iPhone isn't talking to the logic board correctly. This could be the earpiece speaker, front camera, rear camera, proximity sensor, or any other connected component.
Diagnosis requires disconnecting components one by one to find the culprit. A loose flex cable or corroded connector is often the cause.
"Cellular Update Failed" alert
Appears after an iOS update when the cellular modem firmware didn't finish updating. Your phone may not be able to make calls or use mobile data.
Fix:
Go to Settings → General → About
Wait for a carrier update popup — tap Update if it appears
If no popup: Settings → General → Transfer or Reset → Reset Network Settings
If still failing: restart iPhone and check About again
"Apple Pay Requires an Update" alert
The NFC / Secure Element chip firmware didn't complete its update after an iOS update. Apple Pay won't work until it finishes.
Fix: Go to Settings → General → About and wait. The update triggers automatically when you visit this screen. If it doesn't resolve after a few minutes, restart your iPhone and try again. This is a software issue — not hardware.
If you've tried everything above and the error keeps coming back, the problem is most likely physical — something broken inside the device that software can't fix. That's when it needs professional hands-on inspection.