cold water in the washer
Our 336 was 5 years old when we bought it and cushion covers looked it with their dull grey color. We removed covers, using indelible marker to identify which cover goes with which foam (note that the indelible wasn't so indelible). Using cold water, gentle cycle and cold water detergent, not overloading machine and many loads later, we ended up with bright, light blue covers. Did not use dryer, just hung them over the treadmill (finally getting some use). Hardest step was putting covers back on the foam.