Need the model number. Most likely, depending on model, you have two cold valves and one hot valve. One of the cold valve solenoids is wired in parallel with the hot valve solenoid so when hot is selected, you get hot mixed with some cold to "dumb down" the temperature. When you select warm, all three valves are open and when you select cold, only the other cold valve is open. Either the other cold valve is clogged, the solenoid is bad or the temp switch is faulty.