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The cap will not undo, or hisses
The boiler cap will not undo, or hisses
A cap that is hard to undo, or hisses as you turn it, means the boiler is still under pressure or under vacuum. Stop, and follow the manual's order: switch off, open the steam knob, lift the lever into a container, then the cap.
A cap that hisses while the machine is running points to a hardened O-ring.