SIM7670G Modem Stuck on T-SIM7670G-S3 After Voltage Drop (NETLIGHT LED On, 3.8V Present, No AT Response)

[torruella]

Hi LilyGO Community,

I need assistance troubleshooting a LilyGO T-SIM7670G-S3 (ESP32-S3) model 201-220052, board that stopped responding after a network power surge / voltage drop while connected to the cellular network.

Here is the exact diagnostic data and behavior I have gathered so far:

Current Behavior & Measurements:

Power Rail (VBAT): I measured 3.8V directly across the modem's decoupling capacitors. There is NO short to GND on either capacitor (continuity test pings on GND side only, confirming the internal SIM7670G IC is not shorted).

LED Status: The blue NETLIGHT LED turns on (solid), but the modem does not complete boot/registration or blink.

Component Temperatures: No components are heating up (no short circuit draw).

AT Communication: The ESP32-S3 receives no OK response over Serial1 (RX 10, TX 11 at 115200 baud).

Troubleshooting Steps Attempted:

Power & Hardware Pulses:

Pulled BOARD_POWER_ON (GPIO 12) HIGH.

Pulsed MODEM_PWRKEY (GPIO 9) LOW for 1.5s - 2s (and tried HIGH logic active).

Pulsed MODEM_RESET (GPIO 5) LOW for 200ms.

Capacitance / Power Draining: Fully discharged residual gate charges on the PMU lines to rule out latch-up protection.

External Power: Powered via external supply delivering stable 5V / 3A to VIN and verified 3.8V reaching the SIM7670G power rail.

Current Setup / Test Code Used:

C++
#include <Arduino.h>

#define BOARD_POWER_ON 12
#define MODEM_PWRKEY 9
#define MODEM_RESET 5
#define MODEM_RX 10
#define MODEM_TX 11

void setup() {
Serial.begin(115200);
delay(1000);

pinMode(BOARD_POWER_ON, OUTPUT);
digitalWrite(BOARD_POWER_ON, HIGH);
delay(1000);

// Hard reset modem
pinMode(MODEM_RESET, OUTPUT);
digitalWrite(MODEM_RESET, LOW);
delay(200);
digitalWrite(MODEM_RESET, HIGH);
delay(1000);

// PWRKEY Pulse
pinMode(MODEM_PWRKEY, OUTPUT);
digitalWrite(MODEM_PWRKEY, HIGH);
delay(100);
digitalWrite(MODEM_PWRKEY, LOW);
delay(2000);
digitalWrite(MODEM_PWRKEY, HIGH);

Serial1.begin(115200, SERIAL_8N1, MODEM_RX, MODEM_TX);
}

void loop() {
Serial1.println("AT");
delay(500);
while (Serial1.available()) {
Serial.write(Serial1.read());
}
delay(2000);
}
My Questions for the Community:

Has anyone experienced the SIM7670G PMU or internal firmware freezing in a solid-state latch condition where NETLIGHT stays on solid, but UART commands fail?

Is there a specific boot pin / firmware recovery mode (or USB-native SIMCom flashing procedure) for the SIM7670G on this specific board revision?

Could an onboard MOSFET/transistor on the UART TX/RX lines be damaged while leaving the 3.8V rail intact?

Any hardware or firmware recovery suggestions would be greatly appreciated!

Thank in advance