claude-memory/wayback-fan-max-ec-latch.md at main

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Andreas WredeandClaude Fable 5 a304b6d197 memory: initial import (transferred from wayback) + hub-move and cockpit-EMFILE updates

Snapshot of the Pi 'wan' Claude memory: wayback van-router build notes, ZT DNS,
HA VM, LAN port, and the Pi port memory updated for the 2026-07-06 USB hub move
(AP live on the Pi) and the cockpit-bridge fd-limit fix.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

2026-07-06 18:36:43 -04:00

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wayback-fan-max-ec-latch

wayback (Asus UX391U) fan stuck at max = latched EC failsafe; fixed by full power-drain EC reset, not warm reboot

node_type	type	originSessionId
memory	project	0f399db0-9a91-4f55-b102-70875f05be7a

wayback = Asus ZenBook UX391UA (BIOS UX391UA.307, 2020; latest), Ubuntu 24.04, ZFS on zabbly kernel.

On 2026-06-28 the fan ran continuously at max with cool CPU (~50°C idle). Root cause: the Asus EC latched into its fan failsafe, commanding 100% unconditionally. EC temperature sensing was fine (EC byte 0xA0 tracked 43°C→96°C under load); only the fan output was stuck.

Key facts about fan control on this machine:

The OS has NO fan-control authority: no ACPI fan object (PNP0C0B absent, no fan cooling_device), no writable pwm1 (only pwm1_enable on hwmon asus/asus-nb-wmi), asus_wmi fan-curve unsupported (fan_curve_get_factory_default … failed: -19), DPTF INT3400 is mode=disabled (firmware-controlled). So this is never a Linux/driver issue — it's pure EC.
sensors asus-isa-0000 cpu_fan: 25500 RPM is a meaningless 0xFF×100 sentinel from the unsupported asus read path. Ignore it as a diagnostic.

Fix: full power-drain EC reset — shutdown, unplug charger AND all USB-C power, hold power button ~40s, reboot. A warm reboot does NOT clear it (EC stays powered through it). This resolved it; fan quiet at idle afterward.

If it recurs: same power-drain reset. If it returns immediately after a clean EC reset (loud at idle, cool temps) → hardware fault (failing fan or open thermistor), not a latch.

Quick EC inspection: sudo modprobe ec_sys then sudo xxd /sys/kernel/debug/ec/ec0/io (byte 0xA0 = CPU temp °C). rmmod ec_sys when done.

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