Also when checking CO2 on a band A boiler, if it is high, before you adjust it back down first check for any problems like certain boilers develop a crack in the plastic flu collectors which causes mixing POC's in the air intake. Keston flexible exhaust tubes also suffer this as they were the worst materials spec they could have achieved. Ideal Isar and Icos exhaust pipe not made of flexible material like Keston, but just as stupidly made of aluminium so that corrodes, the sign of problems is water damage (which usually gets onto the connector between main pcb and user control hence takes both out). A while back the earlier range of Alpha Band A's were arriving with a cracked plastic flu collector as the heat exchanger was inadequately structurally supported and if transported in wrong orientation it would smash the plastic part. Glow worm and other Vaillant group boilers had massive casing seal problems which they rectified by using graphite seals butt there may be some dodgy ones left. Buderus have changed every single burner and the seals seam to be suspect aswell.
In short, poor flu gas analysis is in the first instance a warning flag for you to look deeper. Band A boilers remove the outer case and repeat flu gas analysis is it is now OK then you know it's leaking poc's into the outer casing seal, so it is one of above issues.
Check flues for seals gine on lost integrity during install by hamfisted installers dislodging the seal. Some manufacturers have had vertical flue terminal problems in the past, and always be aware current models can also have these faults not yet picked up by manufacturer. One simple test is to feel for heat in outer consentric. On the whole there shouldn't be any, check beside joints but also put a temp sensor into intake sensing port which most boilers have.
If all the above checks correct and boiler set up correctly otherwise, now adjust CO2.
