gas water heater chaffoteaux britony 11t (geyser ) 11 years old?

The heater's burners will only come on when the temperature is set at the maximum. On all the other settings it will not fire, the water gets very hot and when I open the cold water on the shower mixer the water goes cold. The pilot lights up without any problem and stays on.
I have changed the diaphragm, the venturi and have cleaned the water governor, the water filter and the gas filter; but the problem is still the same.
The heater was ok last week.