1. Detailed Symptom Description
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LED Behavior: When a 5V/5A power source is connected to USB1, the primary status LED illuminates a dim/low-brightness red for less than one second before cutting out completely.
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No Boot: The device does not transition to a solid red, blinking green, or flashing yellow (EDL) state.
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Thermal/Visual: No visible scorched components or unusual heat were detected, but the rapid power-down suggests a PMIC (Power Management IC) safety trip or a short on a primary power rail.
2. Troubleshooting Steps Attempted
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Power Isolation: Tested with multiple high-quality power sources:
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Dedicated 5V/5A USB-C PD supply.
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Bench power supply (monitored for excessive current draw).
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Connected/disconnected 3.7V LiPo battery.
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Peripheral Removal: All HATs, USB hubs, and HDMI cables were removed; the issue persists on a bare board.
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Recovery Modes:
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EDL Mode: Held power button for 3+ seconds; device fails to enter EDL mode (no flashing yellow).
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USB Detection: Host PC (Windows/Linux) does not recognize the device as a Qualcomm loader or Particle device when plugged in.
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Advanced Debugging: Attempted to access the SysCon UART via the 10-pin debug header using a 3.3V debugger. No serial console output was received, indicating the SysCon MCU is either stuck or not receiving power.
3. Technical Assessment
The "fading red LED" and inability to access the SysCon even via a hardware debugger point to a hardware-level failure in the power management system. This matches community reports where the board's internal safety circuits shut down power before the bootloader can initialize.