Mannion It’s probably the safety valve letting by, one is the vacuum breaker and the other the safety valve next to it, can’t remember which is which.
The vacuum breaker tube will show moisture as it warms up and when the boiler passes 100C shuts and no more bubbling. The safety valve probably won’t bubble until the pressure gets to 2 bar….watch it on the warm up nd it should tell you which tube is which.
If 1 to 0 has no impact at all, then it’s probably the Gicar box that has failed, which is expensive…, check the wiring on the switch. If you do need a new Gicar box, get an up to date one which loses HX and has coffee and x mode coffee. Coffee has very little steam boost after a show for better temp stability…X mode coffee gives the same steam boost as the older machines used to and HX mode no longer exists.
If replacing the Gicar box doesn’t fix it, then you will need to replace the temp sensors.