A pressure indicator for a motor vehicle (or other) fuel tank, that will indicate when pressure at the cap/container interface or vapor pressure in the tank drops below a predetermined level, which will normally mean that the fuel cap is not tightly secured to the tank's opening. The cap/container interface or vapor pressure will be detected by sensors in a rubber gasket (either in the cap or in the tank's opening that the cap fits over) or in a washer for the fuel cap. When the sensors detect a pressure below the predetermined level, BLUE TOOTH (or other appropriate) technology will be used to send a signal to a receiver (preferably in a key ring attachment) that will cause a red light to flash (or other appropriate visual, audio or tactile notice). The light may also flash when the cap is temporarily removed for refueling.