George, this is a classic main module fault. You need to change the main module in the back of the machine. We stock it at £53.88 + £6 postage (exchange). This is ready programmed for your model number. To order, go to our website at www.emwelec.co.uk and put WT540 in the search box (you need the mk4 version) or call us on 01159652118. You will still have the option of returning the module within 14 days for a refund (less our handling of £12) if it does not sort out the problem. Tony EMW Electronics
Tony
January 2015