it looks like your hot water valve for the left side is stuck.
Try to bang it with a mallet, sometimes it helps.
Next : the temperature sensor is defective on that side.
Next: the potentiometer is defective to adjust the temperature
First thing you should try: disconnect the power (the plug on top of the heater valves) from the heater valves. No power = valves are (or should) be completely open). If not, valve is stuck