System locked out; H0 or similar lock code displayed; cannot change settings

Safety: No internal work is required for basic unlocking.

• Unlock sequence: Press and hold the designated lock/unlock button (often a combination like "+" and "-" or a key symbol) for 3–5 seconds until the lock icon disappears. Refer to the Viessmann user manual for the exact sequence.
• Check buttons: If the lock cannot be cleared, check for stuck or damaged buttons on the remote or indoor panel.
• Replace remote or panel: If buttons are physically damaged, replace the remote control or display PCB.