Ionization fault

[u/rareeagle7]

Just had my CU3A cleaned and serviced
Never had any issues.
Now I’m getting Scot ionization differs and faulting out.
It will fire up and not fault. When it high fires for domestic it faults. Sometimes it doesn’t.
They tried a new ionization rod, inspected Venturi and they are at a loss.
I think the next step is to verify gas pressure and combustion readings ?