Another problem leading to compressor failure is regular wear and tear. The compressor runs without being able to create the necessary pressure difference in the system simply because the parts are worn. This can be checked by using the pressure gauges. With the compound gauge connected to the low side and the pressure gauge connected to the high side, if the high-side pressure reads lower than normal and the low-side pressure reads higher than normal, the compressor will have to be replaced as it has lost its compression efficiency. Since it is unable to maintain the required pressure difference in the sealed system, the evaporator temperature never reaches low enough to satisfy the thermostat, causing the unit to run constantly. Note that the evaporator may be covered with a heavy layer of soft frost. An efficient compressor produces a layer of hard frost on the evaporator coil. (As more experience is gained, the evaporator frost pattern will become very evident.) When a compressor is turned off, the evaporator frost pattern disappears very quickly. The frost on the accumulator disappears in few seconds when placing a hand around it. An accumulator in a properly operating system is covered with hard frost.