A theft prevention device essentially consisting of lock members (1) for preventing closure of the valves (4) in diesel and internal combustion engines. Means (6, 7) for operating the lock members (1) and control means (11, 12, 13) therefor are also disclosed. The device is made tamper-proof by a reinforced enclosure (36) and an auxiliary deterrent device (40), and may be used in motor vehicles, motorcycles and agricultural or industrial equipment.
