CH not switching off - Nest/Viessmann

[vikdab]

Hello

I have a NEST with Viessmann system boiler with unvented cylinder.
I have a mid position and two position valve




I thought the mid position valve was faulty as it was not switching off
I got new actuator - the right one (40003916-003/U) . The manuals do make a reference to v4073a1039 so I'm sure it is the right one.

However my CH does not turn off when I switch it off in the app (when there is no HW on).
If HW and CH are on and I turn CH off it does work

I think my NEST is connected ok

App status Nest wiring status Mid posn valve wires

Both off 1245 live Grey live

Ch on 2345 White/Grey Live

HW on 1256 Neither live

Both on 2356 White live

I also notice some spurious voltages for eg after I turn Ch off 3 on nest has 55v (white wire as well)
When both CH and HW are on 4 on nest has 72v (also Grey wire)


Any clues on what is going wrong here

 

[vikdab]

The spring does bring lever back to mid position when there is no power (I switch mains off) so i doubt the spindle is getting stuck.

When I turn power on with CH/HW off on app and force the lever all the way to left (feel spring resistance) the boiler comes on and valve remains open (loose lever)