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Recipe

When an upload fails, in order

Use when: a sketch won't compile, won't upload, or uploads but doesn't work, on an Uno R4 Minima or an ESP32-S3-DevKitC-1

Goal: the one stage that failed, found and fixed, with every earlier stage known to work

Taught in When flashing fails

Quick recipe

  1. Does Verify (compile) succeed?
  2. Is the board's port listed?
  3. Are the right board and port selected, and the port free?
  4. Does the upload still fail to connect?
  5. Uploaded, but nothing happens?
  1. 1

    If not, fix the first error: line. Check that the right board is selected and that every library the sketch includes is installed.

    Why: nothing past this point matters until there is a build.

  2. 2

    Unplug, replug, compare the lists. No new port: try a data cable, another socket, then (bridge chips only) the chip maker's driver.

    • Linux says Permission denied — join dialout (uucp on Arch), log out and in.
    • a native-USB port that was there and vanished after an upload — the sketch has stopped its USB; go to step 4.

    Why: the upload tool needs a port it can open.

  3. 3

    Select them again. Close any other serial monitor.

    Why: a busy port or a stale selection fails the same way as a missing one.

  4. 4

    Force the bootloader. ESP32-S3: hold Boot, press and release Reset, release Boot, upload. Uno R4: double-tap Reset right after plugging in, select the port again, upload.

    Why: a crashed sketch can't hand over to the bootloader, but the bootloader can always be reached by hand.

  5. 5

    Check the wiring and the pin number for this board. Check which port Serial uses (USB CDC On Boot on the ESP32-S3) and the monitor's baud. If the ESP32-S3 waited in download mode, press Reset once.

    Why: a successful upload proves only that the bytes arrived.

Common mistakes

  • Changing cable, port and settings all at once, so the real fix stays unknown.
  • Reading the last error instead of the first.
  • Reinstalling the IDE when the compiler already worked.
  • Assuming a lit power LED proves the cable carries data.