No Boot? Troubleshooting

“No boot” has a narrow set of causes. Work top to bottom; the cheap checks are first.

1. Power really present?

• Measure the ESP32 3V3 pin: 3.3 V.
• Measure BAT+: 3.7-4.2 V.
• Check the battery holder contacts: a loose spring or a reversed cell both read oddly.
• Watch the current draw: a board pulling 500 mA+ is shorted; a board pulling 0 mA has an open power path.

2. Is it booting but silent?

• The OLED may be dead while the ESP32 runs. Connect Serial at 115200 and look for the boot log.
• The red LED on the DevKit (power LED) should light. The onboard “EN” behavior tells you the ROM booted.

3. Flashing state

• A board that was previously flashed may have a corrupted app. Reflash with esptool or the IDE; if upload fails at “Connecting…”, hold BOOT and retry.
• If the board is new and never flashed, it may be running the factory demo or nothing at all. Flash the firmware first.

4. The classic hardware causes

• BUSY pin not wired: firmware hangs in radio init; boot log stops after the radio init line. Fix: wire GPIO 21 to the E22 BUSY.
• Solder bridge on the EN button or RESET line: the board stays in reset. Measure EN: should be 3.3 V.
• Short on the 3V3 rail: radio or OLED wired to the wrong rail. Measure 3V3 under load.

5. Power sag

A dying cell under the PA’s burst can brown-out the ESP32, causing a reboot loop. Symptom: boot screen flashes and restarts. Fix: fresh cell or bench supply; check the BAT+ wiring for voltage drop.

6. Serial output tells you everything

Attach Serial at 115200. The boot log tells you exactly where it stops: flash init, radio init, GPS wait, or the main loop. That single line of evidence eliminates 90% of the guessing.