Diagnostic Techniques

How to identify the problem after a router gets bricked? This page covers common diagnostic methods.

The router is completely unresponsive.

1. Indicator lights are off
2. Cannot ping the router
3. No serial output
4. Usually indicates Flash or Bootloader damage

Some functions work normally, others are abnormal.

1. Indicator lights are on but cannot access the internet
2. Web interface inaccessible
3. Serial may have output
4. Possible firmware corruption

Firmware issue but Bootloader is intact.

1. Can enter Recovery mode
2. TFTP flashing is effective
3. Serial may have output
4. Easiest to recover

Indicator light meanings vary by brand.

Viewing boot logs via TTL Serial is the core of diagnostics.

U-Boot 1.1.4 (Dec  6 2021 - 11:13:09)
CPU: MediaTek MT7621AT
RAM: 256MB DDR3
Flash: W25Q128JV
Hit any key to stop autoboot:  0
## Booting image at 9f020000 ...
  Image Name:   MIPS OpenWrt Linux-5.4.188
  Image Type:   MIPS Linux Kernel Image
  Data Size:    4194304 Bytes
## Loading kernel from 0x9f020000

## Error: "bootargs" not defined
# Boot argument error, needs to be reset

## Error: unrecognized filesystem type
# Filesystem corrupted, need to reflash firmware

## JFFS2: magic bits don't match
# JFFS2 filesystem corrupted

Kernel panic - not syncing: VFS: Unable to mount root fs
# Kernel or filesystem issue

MTD: partition "linux" not found
# Flash partition table corrupted

# View environment variables
printenv

# View Flash info
sf probe

# Test memory
md.l 0x81000000 10

# View boot arguments
print bootcmd
print bootargs

# Test local connectivity
ping 192.168.1.1

# If ping works but Web is inaccessible
# May be a Web service issue, try Telnet
telnet 192.168.1.1

# Windows
tftp -i 192.168.1.1 get test.txt

# If connection works but transfer fails
# Check firewall and ethernet cable

1. Router IP defaults to factory address
2. TFTP server is connectable
3. Web recovery page is accessible
4. Serial output shows recovery mode

Use a multimeter to measure voltages at key points:

1. Power output: 12V/9V/5V DC
2. Key chips on motherboard: 3.3V, 1.8V, 1.2V
3. TTL Serial: 3.3V TX/RX

1. Check capacitors for bulging
2. Check chips for burning
3. Look for cold solder joints
4. Check Ethernet ports for damage

1. Try holding the reset button for 30 seconds
2. Check if the reset button is damaged
3. Measure continuity across the reset button terminals

1. Replace the ethernet cable
2. Disable computer firewall
3. Confirm computer IP settings are correct
4. Try a different TFTP client

1. Confirm firmware matches the model
2. Check if Flash chip is damaged
3. Try clearing NVRAM
4. Reflash via TTL

1. TFTP Recovery
2. TTL Serial Unbrick
3. Firmware Basics
4. Serial Tools