Brownout detector was triggered

Short answer: the ESP32 supply fell below its configured brownout threshold, so the chip reset to prevent undefined behavior. Replace the USB cable, remove unpowered hubs, power the board from a stable source, and disconnect high-current peripherals before flashing again.

Exact message

Brownout detector was triggered

Espressif's fatal-error documentation prints this exact string and says the chip resets after it. Espressif's 2026 ESP-FAQ identifies the root cause as supply voltage falling below the brownout threshold temporarily or continuously; its fixes are a stable supply, a different USB cable, adequate capacitance, and — for battery products — a source able to supply the startup current.

This page was checked on 2026-07-16 against Espressif's ESP-FAQ master release dated 2026-05-25 and the current ESP-IDF brownout documentation.

Fix the power path

1. Use a short, known-good USB data cable and connect directly to the computer.
2. Remove passive hubs, long extensions, breadboards, and external modules for the recovery flash.
3. If the board has a battery, charge it first or disconnect it if the board vendor permits USB-only operation.
4. If you built the hardware, measure the 3.3 V rail at the module during boot and radio startup. A static no-load reading does not prove the rail survives a current transient.
5. Check regulator current capability, ground return, connector drop, and local bulk/decoupling capacitors against the module hardware guide.
6. Reflash only after the board can remain powered without repeating the brownout log.

Do not disable the detector as the first fix

ESP-IDF exposes configuration for the brownout detector and threshold, but disabling it hides the protection rather than correcting an inadequate supply. Espressif says the detector resets the chip to prevent unintended behavior. For a prebuilt board or browser flashing recovery, fix the cable and power source; firmware configuration is only appropriate when the hardware power budget has been measured and validated.

Sources

• Espressif — ESP-FAQ Handbook, section 2.2.19 (root cause and stable supply/cable/capacitance remedies; release 2026-05-25).
• Espressif — ESP-IDF fatal errors: Brownout (exact message and reset behavior; checked 2026-07-16).
• Espressif — ESP-IDF brownout detector configuration (detector purpose and threshold behavior; checked 2026-07-16).
• Espressif — esptool troubleshooting (stable 3.3 V supply and insufficient-power checks during flashing; checked 2026-07-16).