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Upload error · esptool

Fix “getting no sync reply: The serial TX path seems to be down”

The ESP32 entered download mode but never heard esptool. The PC→chip line (adapter TX → RX0) is broken, blocked by a module on GPIO3, or the adapter is dead.

ESP32ESP32-CAMESP8266ESP-01Updated 3 Nov 2026

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What you see

OutputOutput
esptool.py v4.8.1
Serial port COM6
Connecting.....................................

A fatal error occurred: Failed to connect to ESP32: Download mode successfully detected, but getting no sync reply: The serial TX path seems to be down.
For troubleshooting steps visit: https://docs.espressif.com/projects/esptool/en/latest/troubleshooting.html
Failed uploading: uploading error: exit status 2

esptool 4.6 and later can tell which half of the serial link failed. Here the ESP32’s “waiting for download” message reached the computer, so chip → PC works. But the sync packets esptool sent never got an answer, so the PC → chip direction is broken.

On a DevKit, look for something attached to RX0. With a separate USB-TTL adapter, re-check that one wire first.

Quick fix — try in order

  1. 1

    Check the adapter TX → ESP RX0 wire

    With a USB-TTL adapter, adapter TX goes to RX0 (GPIO3 / U0R) and adapter RX to TX0. esptool can hear the chip, so the chip-to-PC wire is fine; the other one is broken, loose or swapped.

  2. 2

    Disconnect anything on GPIO3 / RX0

    A GPS, RS485 module or second microcontroller driving RX0 drowns out the adapter. Unplug it while uploading.

  3. 3

    Set the adapter to 3.3 V logic

    Many CP2102/CH340 sticks have a 3.3 V / 5 V jumper. Use 3.3 V for the ESP; a 5 V TX through a weak resistor divider may not reach a valid level.

  4. 4

    Loopback-test the adapter

    Disconnect the ESP, short adapter TX to RX, open a serial terminal and type. No echo means the adapter's TX is dead — replace it.

Why it happens

How does esptool know the TX path is down?

It reset the chip and read the ROM boot message saying “waiting for download”, so chip → PC works. But the chip never replied to esptool's sync packets, so PC → chip does not.

Dev board with a built-in USB chip

On a DevKit the trace is on the PCB, so the usual cause is something else connected to RX0, or a damaged USB-serial chip after a short on the 5 V rail.

Board-specific notes

ESP32-CAM: With an FTDI-style adapter, power from 5 V but keep the adapter's logic jumper on 3.3 V. U0R on the CAM is the RX pin.

ESP-01: RX is the pin next to 3V3 on the 2×4 header. Swapped RX/TX is the classic mistake.

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