@dc42 This is what I get when I run M122 with the setup you mentioned. It looks like Vin is normal (~24V):

M122
=== Diagnostics ===
RepRapFirmware for Duet 2 WiFi/Ethernet version 2.05.1 running on Duet WiFi 1.02 or later
Board ID: 08DLM-996RU-N85T0-6J9F6-3SJ6J-KAVHR
Used output buffers: 1 of 24 (1 max)
=== RTOS ===
Static ram: 25712
Dynamic ram: 92704 of which 0 recycled
Exception stack ram used: 312
Never used ram: 12344
Tasks: NETWORK(ready,1876) HEAT(blocked,1232) MAIN(running,4520) IDLE(ready,160)
Owned mutexes:
=== Platform ===
Last reset 00:00:38 ago, cause: power up
Last software reset details not available
Error status: 0
Free file entries: 10
SD card 0 detected, interface speed: 20.0MBytes/sec
SD card longest block write time: 0.0ms, max retries 0
MCU temperature: min 22.2, current 25.9, max 26.0
Supply voltage: min 24.1, current 24.1, max 24.2, under voltage events: 0, over voltage events: 0, power good: yes
Driver 0: standstill, SG min/max not available
Driver 1: standstill, SG min/max not available
Driver 2: standstill, SG min/max not available
Driver 3: standstill, SG min/max not available
Driver 4: standstill, SG min/max not available
Date/time: 1970-01-01 00:00:00
Cache data hit count 58077191
Slowest loop: 0.21ms; fastest: 0.07ms
I2C nak errors 0, send timeouts 0, receive timeouts 0, finishTimeouts 0, resets 0
=== Move ===
Hiccups: 0, FreeDm: 160, MinFreeDm: 160, MaxWait: 0ms
Bed compensation in use: none, comp offset 0.000
=== DDARing ===
Scheduled moves: 0, completed moves: 0, StepErrors: 0, LaErrors: 0, Underruns: 0, 0
=== Heat ===
Bed heaters = 0 -1 -1 -1, chamberHeaters = -1 -1
=== GCodes ===
Segments left: 0
Stack records: 0 allocated, 0 in use
Movement lock held by null
http is idle in state(s) 0
telnet is idle in state(s) 0
file is idle in state(s) 0
serial is ready with "M122" in state(s) 0
aux is idle in state(s) 0
daemon is idle in state(s) 0
queue is idle in state(s) 0
autopause is idle in state(s) 0
Code queue is empty.
=== Network ===
Slowest loop: 0.20ms; fastest: 0.00ms
Responder states: HTTP(0) HTTP(0) HTTP(0) HTTP(0) FTP(0) Telnet(0) Telnet(0)
HTTP sessions: 0 of 8

WiFi -
Network state is disabled
WiFi module is disabled
Failed messages: pending 2779096485, notready 2779096485, noresp 2779096485
Socket states: 0 0 0 0 0 0 0 0
ok