---
name: wayback-fan-max-ec-latch
description: "wayback (Asus UX391U) fan stuck at max = latched EC failsafe; fixed by full power-drain EC reset, not warm reboot"
metadata: 
  node_type: memory
  type: project
  originSessionId: 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.