Originally Posted by
xliioper
Z fares have U/T/X/V fare basis codes (and thus require at least U to be open to book), while I fares have Q/K/L (although cert allows either Z or I fares so having I and Q open should be sufficient for cert as long as any fare rule requirements are met). Not clear if poster was trying to use certs for coach or FC. Coach requires L/U/T/X/V booking availability and it may never open for popular flight times.
In addition to EF, you can set Google Flights price alerts (can't set for a specific fare bucket, but if you are familiar with fare filings on route you can see if price point fall to point which indicates availability in fare bucket).
Looks like not quite the case for I fares (at least my current expertflyer shows thusly):
9/10/22 DEN - ALB:
DL712 (DEN-ATL) shows:
J9 C9 D9 I9 Z8 W9 S0 Y9 B9 M9 H9 Q9 K9 L5 U0 T0 X0 V0 E9
DL2225 (ATL-ALB) shows:
J9 C9 D9 I9 Z9 W9 S0 Y9 B9 M9 H9 Q9 K9 L4 U0 T0 X0 V0 E9
Delta.com won't show it as eligible for First Class (I or Z) with companion cert.
Z, I can understand if it requires UTXV since there's no UTXV available. But plenty of IQKL available.