Lightning bolt under-voltage warning keeps appearing on Raspberry Pi 4 despite 5V 3A supply

Understanding the Under-Voltage Trigger

The Raspberry Pi 4 triggers an under-voltage warning when the voltage on the 5V rail drops below approximately 4.63V to 4.65V. While your power supply may be rated for 5V 3A, the voltage measured by the SoC can be significantly lower due to voltage drop (V=IR) across the USB-C cable and internal traces. This is why a high-amperage supply can still trigger a warning if the cable resistance is too high.

Analyzing vcgencmd get_throttled

The vcgencmd get_throttled command provides a bitmask that explicitly distinguishes between current and historical power events. This allows you to determine if you are dealing with a constant deficit or transient spikes.

Bitmask Interpretation

The output is a hexadecimal value. To analyze it, convert the hex to binary and check the following bits:

Bit	Meaning	State
0	Under-voltage detected	Current
16	Under-voltage has occurred	Since Boot
2	ARM frequency capped	Current
17	ARM frequency capped has occurred	Since Boot

Transient Dips and Hysteresis

Transient voltage dips—caused by sudden peripheral load spikes—can be short enough to avoid triggering the on-screen lightning bolt icon but are still captured by the firmware. If vcgencmd get_throttled returns a value where Bit 16 is set but Bit 0 is not, a transient dip occurred. There is no widely documented software-configurable hysteresis for this hardware trigger; it is a fixed firmware threshold designed to protect the SoC from instability.

Verification Steps

1. Check the current state: Run vcgencmd get_throttled. If the result is 0x0, no under-voltage has occurred since boot.
2. Identify historical dips: If the result is 0x10000, an under-voltage event occurred, but the system is currently stable.
3. Isolate the cable: Replace the USB-C cable with a shorter, thicker-gauge (lower AWG) cable to see if the historical flag (Bit 16) stops appearing during peripheral activity.

Missing Diagnostic: Are you using a dedicated power supply or powering the Pi via another device's USB port? This significantly changes whether the issue is cable resistance or a current-limiting source.