Abstract: A valve cap is screwed onto a pneumatic tire and generates an indication whether a prefilled tire pressure is still existent or not. For this purpose a valve cap (1) comprises a deflectable diaphragm (30) in an inner housing separating a pressure chamber from a reference pressure chamber. The reference pressure chamber registers the prefilled tire pressure. The diaphragm (30) is deflected toward the closed housing end when the actual existent tire pressure drops in comparison with the registered tire pressure. Onto the diaphragm (30) a permanent magnet (40) is placed, which can be in a first position (normal position of the diaphragm) or in a second position adjacent to the housing end. The check of the magnet position and the generation of an indication signal indicative of the actual tire pressure status will be executed by a hand device, which must be brought into a defined position with respect to the valve cap.