Incorrect behavior of Asuswrt-Merlin and inability to restore stock firmware on Asus RT-AC66U B1

[network_user1]

Hello.

I need help and advice on how to recover my router. I am not a hardware engineer or network administrator — just a regular user.


1. Background

About a year ago someone gave me a non-working ASUS RT-AC66U B1 router (I also own another RT-AC66U B1 running FreshTomato, that one works fine).

The "broken" router behaved like this:

• LEDs reacted to cable insertion in every port except LAN2
• The router did not establish network connections
• When connecting it to other routers, the link speed was detected incorrectly
• (10 or 100 Mbps instead of 1 Gbps), or there was no link at all
• Wi-Fi did not appear
• It could not enter Rescue mode — Power LED did not flash

At that time I gave up on it.


2. Recent progress

Recently I found it again while sorting old equipment and decided to power it up. Surprisingly, it booted — slowly, but it worked.

I reset it to factory settings (because I didn’t know the passwords) and upgraded the firmware from Asuswrt-Merlin 386.12 to 386.14_2 (with full reset)

Router now:

• boots and operates
• Wi-Fi networks appear
• routing under load works normally
• CPU temperature stays around 58–62 °C (under normal load)

But:

• port LAN2 is still dead
• Merlin firmware uses only one CPU core instead of two (visually confirmed in System Monitor and via /proc/cpuinfo)

ASUSWRT-Merlin RT-AC68U 386.14_2 Sun Nov 17 19:17:59 UTC 2024
admin@RT-AC66U_B1-C780:/tmp/home/root# cat /proc/cpuinfo
Processor       : ARMv7 Processor rev 0 (v7l)
processor       : 0
BogoMIPS        : 1998.84
Features        : swp half thumb fastmult edsp
CPU implementer : 0x41
CPU architecture: 7
CPU variant     : 0x3
CPU part        : 0xc09
CPU revision    : 0

Hardware        : Northstar Prototype
Revision        : 0000
Serial          : 0000000000000000
admin@RT-AC66U_B1-C780:/tmp/home/root#

3. Goal

I want to restore stock ASUS firmware (the official firmware for this model still receives updates in 2025) and hopefully get both CPU cores working properly again.


4. Recovery attempts

I successfully enter recovery mode (the power LED is blinking):

ping 192.168.1.1
Pinging 192.168.1.1 with 32 bytes of data:
Reply from 192.168.1.1: bytes=32 time=3ms TTL=100

TTL=100 → recovery mode, correct.

Downloaded the official firmware RT-AC66U_B1_3.0.0.4_386_52062-g06fe188.trx . Renamed it to: asus.trx

Attempt #1 — Asus Firmware Restoration Tool

Result: “The wireless router is not in rescue mode.” Even though Power LED is flashing and ping says TTL=100.

Attempt #2 — CFE MiniWeb Server

When opening http://192.168.1.1 (Firefox, Microsoft Edge) I get: ASUSTek – CFE miniWeb Server. But after selecting the firmware and pressing Upload, the browser loads for a while and then URL changes to: http://192.168.1.1/f2.htm "The connection was reset while the page was loading"

I waited 10–15 minutes, rebooted manually → still boots into Asuswrt-Merlin.


Attempt #3 — TFTP upload

Pinging 192.168.1.1 with 32 bytes of data:
Reply from 192.168.1.1: bytes=32 time=1ms TTL=100
Reply from 192.168.1.1: bytes=32 time=2ms TTL=100
Reply from 192.168.1.1: bytes=32 time=4ms TTL=100
Reply from 192.168.1.1: bytes=32 time=4ms TTL=100

4:55 tftp -i 192.168.1.1 put asus.trx
5:05 Transfer successful: 106954772 bytes in 592 second(s), 180666 bytes/s

After upload:

• Power LED turns off
• LAN1 LED flashes very fast
• No response to ping

ping 192.168.1.1
Pinging 192.168.1.1 with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.
Ping statistics for 192.168.1.1:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss)

I waited 20-30 minutes. After manual reboot → Merlin loads again. I tried this method few times.


5. Question

What else can I try to successfully flash the stock firmware?
Any guidance or suggestions are appreciated.
Thank you.


Below are the logs from the router web UI after factory reset and reboot.

 

[network_user1]

I can't post the logs. Attaching a file doesn't post the file to the thread. And trying to post a comment with the log text returns an error:

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Oops! We ran into some problems. Please try again later. More error details may be in the browser console.

I posted logs here : https://pastebin.com/eF2Yg7qS