Most original EFIS tubes include older tube technology with a high voltage power supply (HVPS) that heats a CRT (cathode ray tube). Because units with CRTs can operate at high voltages they can be susceptible to significant failure. When failures in the power supply occur, they usually must be replaced. The failure can lead to problems in the CRT assembly itself as well. In most cases it is most cost effective to replace the unit with another serviceable unit when either the CRT or HVPS fail. In addition, the CRT is subject to screen burn in and needs to be replaced in order to be certified for use on an aircraft. Most manufacturers allow around 25% burn in before the CRT must be replaced.