@chrishamm Both power supplies have their neutrals connected and the machine is earthed.
I put a Multimeter on the 24v supply for the Mini5plus. When I turned the heats on the voltage dropped to 23.7v but the machine didn't fault, as soon as i removed the meter it faulted.
I will get a new PSU tomorrow and see if it makes a difference.
It could be a coincidence the printer was running fine on the old firmware before updating to 3.5.0

This is how the 6HC and The SBC are connected

After updating to the new firmware my printer now has this error.

This happens when the machine is cold and I turn the nozzle and bed on together, I could get around this by heating them up 1 at a time but now it stops mid print.
The problem started after upgrading to 3.5.1 and was whenever i turned the bed on. I changed the bed to another output via a relay, this fixed it for a little while and allowed me to print.
Now i have upgraded to 3.5.2 and it is stopping the machine mid print.
If i try to send M122 nothing happens if i send M112 It resets the fault.

Product Short Name Version
Duet 3 MB6HC MB6HC 3.5.2 (Running 48v for stepper motors)
Duet 3 Expansion TOOL1LC TOOL1LC 3.5.2
Duet 3 Expansion TOOL1LC TOOL1LC 3.5.2
Duet 3 Expansion SZP SZP 3.5.2
Duet 3 Expansion Mini5plus Mini5plus 3.5.2 (Running 24v for heating and sensors)
Duet Software Framework DSF 3.5.2
Duet Web Control DWC 3.5.2

Thanks
Phill

@jay_s_uk That fixed it thanks.

I updated my firmware via the sbc mode and now I get an Incompatible software versions error on startup.
Is it possible to update DWC separately in sbc mode?

Product Short Name Version
Duet 3 MB6HC MB6HC 3.5.0-rc.2
Duet 3 Expansion TOOL1LC TOOL1LC 3.5.0-rc.2
Duet 3 Expansion TOOL1LC TOOL1LC 3.5.0-rc.2
Duet 3 Expansion SZP SZP 3.5.0-rc.2
Duet 3 Expansion Mini5plus Mini5plus 3.5.0-rc.2
Duet Software Framework DSF 3.5.0-rc.2
Duet Web Control DWC 3.5.0-rc.1

Thanks
Phill