Migrating from nest 2 thermostat to nest 4

[DBealby]

My nest thermostat gen 2 works fine but will soon be obsolete. I’m migrating to nest gen 4. The next gen 4 doesn’t recognize any power on arc. When I install it back to nest gen 2 it works fine. 
any suggestions?

btw I have two furnaces thus 2 thermostats. I’ve mixed and matched with no success 

[DBealby]

That should be no power on Rc

[Patrick_Caezza]

@DBealby 

How are the 2nd Gen units wired?

Can you post pictures of both?

[DBealby]

 Patrick_Caezza

[DBealby]

I got the gen 4 thermostat working. Rc wire was not 100% clean, so it was good enough for gen 2 but not clean enough for gen 4.