It Won't Arm

The single most useful fact in this page: Betaflight always knows why it is refusing to arm, and it will tell you. Connect USB, open the CLI, type status — the Arming disable flags: line lists every active reason by name. The configurator shows the same list on the Setup tab. Never guess; read the flags.

The common flags, decoded

Flag	It means	Fix
RXLOSS	The FC hears no receiver	Radio on? Bound? Ports tab UART right? — ELRS binding and Betaflight basics
ANGLE	Tilted beyond small_angle (default 25°)	Put it flat; if it is flat, recalibrate the accelerometer (Betaflight basics)
THROTTLE	Throttle channel isn't at its minimum	Check endpoints on the Receiver tab read ~1000 at stick-low (Radio Transmitter)
ARM_SWITCH	The arm switch was high at boot	Toggle it low then high — a safety feature, not a bug
MSP / CLI	A configurator/CLI session holds the board	Disconnect USB / type exit — bench-arming happens props-off anyway (Module 10)
FAILSAFE	The failsafe stage is engaged	Almost always link quality at the bench — antenna, distance to the radio (Receiver)
BOOTGRACE	You tried within ~5 s of power-up	Wait — gyro calibration wants stillness
CALIBRATING	Gyro still calibrating	Don't move the quad at plug-in

If the flags say nothing

• Arm switch assigned at all? Modes tab, range actually covering the switch's output (Betaflight basics).
• Right model selected on the radio, and model match not pointing at a different quad (Radio Transmitter)?
• After a crash: RPMFILTER or a motor refusing its idle spin can hold arming on bidirectional-DShot builds — check motor problems.

The flags are the method

This page could be ten times longer and still incomplete — flags get added with firmware versions. The transferable skill is the first box of the tree: the FC tells you why. status first, forums second.