//If their supposed reading is correct why is my reading 88888.888 wrong ?//
There is a fault with the digital readout on your meter. It's nothing to do with valves. Your meter has a seven-segment Liquid Crystal Display ("LCD"). You will note that each digit has seven strokes available for use to make up the numbers zero to nine. All seven are displayed when the figure eight is required. All seven are also sometimes displayed when the device is faulty. If every digit is an "8" there is almost certainly something wrong with the display mechanism.