Editing Openai/692da612-aac4-800b-80dd-f1568632e2ff (section)

=== This means the MCU might not be running user code (odd, since flash was written). Try these in order: ===

A) Press the RESET button once immediately after upload finishes
* Immediately after you see avrdude done, press RESET and watch Serial Monitor.
* Do you get START_TEST printed? If yes — auto-reset circuit (DTR capacitor) may be faulty. You can press RESET manually each time or fix circuit.

B) Check RX/TX LEDs behavior during upload
* Watch board LED labeled TX/RX while uploading — do they flash? - If they flash during upload, PC talked to board (we know upload works). - If they never flash, but avrdude still said Done — weird; still continue.

C) Power-check — measure board voltages (use multimeter)
* Measure 5V between 5V pin and GND → should read approx 5.0 V.
* Measure 3.3V if present (optional). If 5V is missing or very low, MCU can't run even though flash was written.

D) Check ATmega328P presence / seating
* If your board uses a removable DIP ATmega328P (IC in a socket), power OFF, gently remove the chip and reseat it (careful about orientation notch).
* If SMD chip (surface mount), visually inspect for cracked solder joints.

E) Try Reset while Serial Monitor is closed then open Serial Monitor after a second — sometimes serial console timing hides output.

Report the results for A–E you tried.