The lightning bolt means the supply voltage fell below the safe level. Use a proper supply and a short, thick cable.
What the message means
A Pi needs steady 5 V and enough current. Thin cables and weak chargers drop the voltage, and the Pi may slow down, lose Wi-Fi or corrupt the card.
Fix it in this order
- Use a supply made for your Pi model and its rated current.
- Use a short cable with thick wires.
- Measure the voltage on the test points if your board has them.
- Unplug USB devices that draw much current, or use a powered hub.
- When on a battery, use a good regulator rather than a plain charger lead.
- Check that servos and motors are not on the same supply.
Why it happens
The lightning bolt icon appears when the Pi measures its supply below about 4.63 V. The usual cause is a weak charger, a thin or long cable, or a load such as a camera, a USB device or motors on the same supply.
Use the official power supply for your model, a short good cable, and power motors and servos from a separate supply with a shared ground.
If you see this, do this
| You see | Do this |
|---|---|
| Warning appears under load | Supply or cable too weak. |
| Random freezes | Check power first. |
Related
ESP32 brownout, power bank cut-off.
Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.
Why this happens
The Pi checks its 5 V input and shows the lightning bolt when it falls below about 4.63 V. The usual cause is not the charger label but the voltage lost on the way. A cable has resistance, and the voltage lost is current times resistance. A cable with 0.25 ohm round trip carrying 3 A loses 0.75 V, so 5.1 V at the charger becomes 4.35 V at the board.
The load can also jump suddenly. Starting a camera, plugging in a USB drive or switching a motor on the same supply pulls the voltage down for a moment, which is enough to trigger the warning.
How to confirm the cause
- Run vcgencmd get_throttled. A value of 0x0 means no problem has been recorded since boot.
- Read the bits. Bit 0 is under-voltage now and bit 16 means it has occurred since boot. Bit 2 is currently throttled and bit 18 is throttling has occurred.
- Run dmesg and search for the under-voltage message with the time it appeared.
- If your board has test points, measure the voltage between them while the Pi works hard. Below about 4.75 V is a problem.
- Unplug USB devices one at a time and watch whether the warning stops.
Worked example
Decoding a result from vcgencmd get_throttled, and the cable loss calculation.
vcgencmd get_throttled
throttled=0x50000
0x50000 = bit 16 + bit 18
-> under-voltage has occurred, throttling has occurred, neither is happening now
drop = I * R = 3 A * 0.25 ohm = 0.75 V
5.1 V - 0.75 V = 4.35 V at the board (too low)Common mistakes
- Using a phone charger without checking that it can supply the current your model needs.
- Powering servos or motors from the Pi’s 5 V pin.
- Using a long, thin or old cable.
- Feeding 5 V into the GPIO pins with thin wire, which bypasses the protection and the drop is larger.
- Ignoring the warning because the Pi still boots.
If this did not fix it
- A power bank that cuts out or does not deliver full current on its port.
- A failing SD card, which causes freezes that look like power faults.
- A hot Pi. Temperature throttling is a different warning from under-voltage.