Hi,
This 'Sync timeout' error means the ESP chip is refusing to enter 'bootloader mode' (the mode needed to flash the firmware).
Here are the best ways to fix it:
- Force Bootloader mode manually
If your Denky D4 has a BOOT button: Press and hold it.
While holding the BOOT button, press the Reset button once (or plug the USB cable in).
Release the BOOT button after 1 or 2 seconds.
Try the Web Flasher again.
(Note: If you don't have a BOOT button, you have to use a small wire to connect the GPIO0 pin to GND while plugging the USB in).
-
Use a better flashing tool
Web browsers sometimes struggle with the ESP timing. Instead of the Web Flasher, download a standalone software called ESP-Flasher. It is usually much more reliable. -
Check your hardware
Make sure you are using a high-quality USB data cable (some cables only provide power).
Plug the USB cable directly into your computer's motherboard, avoiding USB hubs or extension cords.





