Hi
The hot -- cold control operates a tap, cock or faucet, to control the rate of flow of water through the heater matrix.
The cock gets tight to turn e.g. from hard water & corrosion and the wire between the control and the tap bends instead of operating the tap to control the rate of flow of water.
Remove the sound insulation blanket from the bulkhead engine side and you may be able to inspect the problem, replacing the cock may be all you can do...
Noel